Parents should see to it their children form the habit of brushing their teeth every day.

问题补充：

Parents should see to it their children form the habit of brushing their teeth every day.

A. this

B. that

C. whether

D. if

答案：

A. this

(答案→)B. that

C. whether

D. if

解析：考查从句，that引导的同位语从句，对it进行解释说明，句子意思“父母应该料理他们的孩子天天刷牙的习惯”