This is my theory of the answer (the bottom says 105,200) By the way
If this is biology then the answer is Enzyme. It pairs with the complementary substrate to make an Enzyme-substrate complex.
Answer: The ACT is a standardized test that measures a student's skills in five core areas: English, math, reading, science, and writing (optional). Students in grades 11 and 12 take the ACT so that they can submit their scores to colleges as part of the college application process. 75 questions in 45 min.