There _____ a pair of socks in the box.

问题描述:

There _____ a pair of socks in the box.

a pair of 这样的,要根据pair的单复数来决定谓语动词
所以用is
绝对是单数的
如果主语用a kind of ,a pair of ,a series of等加名词构成时,谓语动词一般用单数形式.A pair of shoes was on the desk.