the school, he saw his parents for him at the school gate.
A. At reaching; waited
B. On arriving; waiting
C. At arriving; waited
D. On reaching; waiting
网友回答
A. At reaching; waited
B. On arriving; waiting
C. At arriving; waited
(答案→)D. On reaching; waiting
解析：考察介词用法以及固定结构。介词on一…就…；后面要使用动名词doing形式。第二空考察了see sb doing sth看见某人正在做某事。句义：一到学校，他就看见父母亲在校门口等着他。故D正确。