---How long did it you to work out the difficult maths problems?---I about half an hour

问题补充：

---How long did it you to work out the difficult maths problems?

---I about half an hour on it .

A. take ; took

B. spend ; took

C. spend ; cost

D. take ; spent

答案：

A. take ; took

B. spend ; took

C. spend ; cost

(答案→)D. take ; spent

解析：句意：——你需要花费多长时间解决那些数学难题？——我花了大约半个小时。It took sb. to do sth；做某事花费某人多长时间。sb. spent sometime on sth./in doing sth. 某事/做某事花费某人多长时间。；sth. cost sb. some money. 某物花了某人多少钱。故选D。