It is widely believed that lancelets—small, primitive sea animals—do not have hearts. ████ ████████ ███ █ ███████████ ███████ ███ ████ ██████ ██ ██████████ ██ ██████ ██████ ████ █ ██████ ████████ ████ ██████ ██ ██████ █ ██████ █████ ████ ██ ████████ █████████ ███ █████████ ██ ███ █████ ██ ███████ █████ ███ ████████ █████████ ███ ████████ ████████████ ██ ███ ██████████ ██████ ███████ ████████ ███ ████████ ████████████ ██ █████ ████████ ███████
The author concludes that lancelets have hearts, because they have a vessel that is similar to other hearts in their structure and contractions.
The conclusion is about having a heart, but we aren’t given information on what qualifies as having a heart. The support only comes from the structure and behavior of the lancelet’s vessel bearing similarity to other hearts. We can make the argument valid if we know for sure that one or both of these qualities (structure or behavior) having commonality with other hearts is enough to qualify as having a heart.
The argument's conclusion follows logically ██ █████ ███ ██ ███ █████████ ██ ████████
Only animals that ████ ███████████ ███████ ████ ███████
This is switching sufficient and necessary. (A) can only support a conclusion about having contracting vessels or, through contrapositive, not having a heart. We need something that would support a conclusion about having a heart.
Some primitive animals █████ ████ █████████ ████ ████ ██ ██████ ████ ██ ██ █ ██████
Irrelevant. Just because other animals with one similarity to the lancelet have a heart, it does not strengthen the argument that lancelets have a heart. Even in a Strengthen question, this answer is problematic.
A vessel whose █████████ ███ ███████ ███████ ████████ █████ ██ █████ ██████ ██████ ██ █ ██████
This links the vessel of the lancelet to a conclusion about having a heart. This means the traits of the lancelet’s vessel qualify for having a heart.
For a vessel ██ ██ ██████ ██ ██ ████████ ██████████ █ ██████ ████ ██████ ████ ███████ ████████ █████████████
Similar to (A), a reversal of what we need. We need for “properly considered a heart” to be a necessary condition. We need to conclude that something has a heart, and we cannot do that when it is in the sufficient condition.
No animal that ███ █ █████ █████ ██ ███████
Phrased another way, this is “If an animal has a heart, then it has an artery.” This has the same issue as (A) and (D)— We need to conclude that something has a heart, and we cannot do that when it is in the sufficient condition.
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