I think it is 3 i'm not sure but i'm about 70% sure
The correct answer is Envelope
Using energy is the answer to this question :)
Crossing over occurs at the four-strand stage when two homologous chromosomes, each consisting of a pair of sister chromatids, are paired. Each crossover involves just two of the four strands and generates two recombinant strands.
Hope this helps!
Answer:
because protein molecules are very large in molar masses