They usually go to see their grandparents Sundays.A. ofB. inC. onD. at

问题补充：

They usually go to see their grandparents Sundays.

A. of

B. in

C. on

D. at

答案：

A. of

B. in

(答案→)C. on

D. at

解析：句意： 他们通常在星期天去看爷爷奶奶。在具体的日期前应用介词on。故选C。