Parentssend their children to school when they reach six years old.A. couldB. were able to

问题补充：

Parentssend their children to school when they reach six years old.

A. could

B. were able to

C. ought to

D. would

答案：

A. could

B. were able to

(答案→)C. ought to

D. would

解析：句意：当孩子满六岁时，家长应当送孩子去上学。根据语境可知，应填ought to，表示有责任，从法律或道义上“应该”做。