Why does this not work for number 4?

1. current instruments vs more sophisticated instruments

2. which can detect planets outside our solar system?

3. more sophisticated instruments

For question one, is this also a valid answer:

There is a smaller surplus of corn seeds this year than there was last year.

1. surplus of corn seeds this year vs surplus of corn seeds last year

2. which is smaller?

3. winner: the surplus of corn seeds this year

Q1 was a repetition from the previous skill builder, but it had a different answer. The previous skill builder said that "it" in the first sentence was a referential. Why is that not correct now?

How do you know when something is an object vs when it is a modifier of the predicate verb?