Self-paced
Small experimental vacuum tubes can operate in heat that makes semiconductor components fail. ███ █████████ █████ ██████████ ██ ████ ██ ███████ ████ ████ ██ ██████████████ █████ ██ ██████████ ███ ███ ██ ███████ █████████ ███ ████ ██ ████ █████████ ████ ████ ██████████ ██ ██████████████ ██ ███ █████ ███████████ █████████ ████ ██ ███████ ███████ █████████ ████████ ██████ ██████ ███████ ███████ ████████ ██ █████████ ███ ██████████ ██ ████ ██ ███████████████
Stimulus Summary
For a component with resistance to heat greater than semiconductors to be preferable to semiconductors in digital circuits, it needs to be comparable to semiconductors in all other significant respects.
This means that if a component is not comparable to semiconductors in all other significant respects, then the component cannot be preferable for use in digital circuits.
Anticipation
Vacuum tubes resist heat better than semiconductors, since they keep working in heat that makes semiconductors fail. That means the rule in the second sentence applies to them. According to the rule, in order to be preferable, the vacuum tubes must be comparable to semiconductors in all other significant respects.
The third sentence says vacuum tubes' maximum current capacity is not comparable to that of semiconductors. Maximum current capacity is one of those significant respects. So vacuum tubes fail the requirement, and therefore they cannot be preferable for use in digital circuits right now.
6.
If the statements above are █████ █████ ███ ██ ███ █████████ ████ ████ ██ █████
a
Vacuum tubes are ███ ███ ██████████ ██ ██████████████ ███ ███ ██ ███████ █████████
(A) must be true. For a heat-resistant component to be preferable for digital circuits, it has to be comparable to semiconductors in all other significant respects, and maximum current capacity is one of those respects. Vacuum tubes are not comparable right now, so they fail the requirement for being preferable. That makes them not now preferable, which is what (A) says.
b
Once vacuum tubes ███ ██████████████ ████ ██████████ ███████ ███████ █████████ ██████ █████ ████ ██ ████ ██ ████ ███████ █████████
Matching semiconductors on maximum current capacity is only one of the "all other significant respects" the rule demands, so closing that one gap would not guarantee vacuum tubes become preferable, since some other respect could still fall short. And being preferable is not the same as being used. The stimulus is about preferability, not about what actually gets used, so "will be used in some digital circuits" goes past anything we know.
c
The only reason ████ ██████ █████ ███ ███ ███ ████ ██ ███████ ████████ ██ ████ ██████ ██████ ███████ ███████ ████████ ██ ███ ████
(C) overreaches with "the only reason." The stimulus describes what makes a component preferable, not why vacuum tubes are or aren't used, and it offers maximum current capacity merely as one example of a significant respect. Other respects could fall short too, so we cannot call low maximum current capacity the single reason for anything.
d
Semiconductors will always ██ ██████████ ██ ██████ █████ ███ ███ ██ ████ ████████████ █████ ████ ███████ █████████
The stimulus is entirely about digital circuits and says nothing about other applications or about what will hold "always." Whether semiconductors stay preferable for other uses is outside what we were told.
e
Resistance to heat ██ ███ ████ █████████ ████ ██████ █████ ████ ████ ███████████████
We know vacuum tubes resist heat better and that their maximum current capacity is worse, but nothing rules out other advantages they might have. "Only advantage" is not something the stimulus supports.