New SAT: How to answer this type of grammar question?

I have seen this twice on one grammar test so far and got it right twice, but I don’t know why. The question is “Which choice most effectively combines the two sentences at the underlined portion?” All the answer choices are grammatically correct…

Can you give us an actual problem, it would make explaining it much more effective?

@mklaben15 They might all be grammatically correct, but it’s likely that one fits best within the context of the passage.

https://collegereadiness.collegeboard.org/pdf/sat-practice-test-1.pdf questions 13 and 39. @elena3142

Watch out–they like to try to trick you into creating a fragment, so an answer choice will put a period after a clause, then show you the first word of the next part, but if you check the passage, that will make it a fragment.

You want to be brief but comprehensive. Within the context of the sentence, do not say more than what is necessary.

For 13, B, C, and D contains redundant information.

Likewise for 39, you can eliminate B, C, and D.