—Where is our English teacher?

—She is in the classroom, _____ the exercises _________ the students have done at home.

A．explaining; /

B．explaining; for

C．explained; to

D．explained; from

钢材的堆放高度应达到所堆宽度的3倍，否则，钢材堆放的高度不应大于其宽度。堆放时，一般应一端对齐，并在前面竖立标牌，写清工程名称、钢号、规格、长度、数量。