--- Had we been more careful, the accident might not have happened.

--- But we _____ at that time.

A．hadn’t

B．didn’t

C．weren’t

D．might not

国家规定，所有适龄儿童、少年都要入学接受规定年限的义务教育。这体现了义务教育的哪一特点：[ ]

A、公益性

B、统一性

C、强制性

D、免费性