Read the paragraph.

[1] Tia and Jackie are partners in science class. [2] They have a big project to complete and are having trouble agreeing on an idea. [3] Jackie would prefer to study geology, but Tia is more interested in human biology. [4] Ultimately, they decides to study fossilization.

Which sentence has a subject-verb agreement error?
sentence 1
sentence 2
sentence 3
sentence 4