they each (answer) the question one by one.they 后的each是对they起强调作用的,但是主语还是they,所以动词用原形.
还有,one by one 是修饰动词answer的副词,副词不能决定动词用原形还是第三人陈单数的形式,只有动词的主语才能决定动词的单、复数形式.