Scientists chose twins for their study because x

* Hướng dẫn giải

Đáp án là A. Dựa vào đoạn 4: One of the twins received a normal upbringing, and performed well. The other twin, however, was brought up by extremely supportive parents and given every possible opportunity to develop its abilities. That twin, though starting out with the same degree of intelligence as the other, performed even better.