The students came back to their classroom in?

B.twos and threes
twos and threes “三三两两” 固定搭配没什么可解释了.
by/in twos and threes 三三两两
in threes and fours 三五成群
at sixes and sevens 乱七八糟
1.They came home from church by twos and threes.
他们三三两两地从教堂回来.
2.They sat in twos and threes on the trim barbered lawn.
他们三三两两地坐在修剪平整的草坪上.

c