问题
选择题
Being a teacher always encouraging students to think for themselves, I won't tell them answers until they __________ on a math problem for more than an hour.
A．have worked
B．will have worked
C．will have been working
D．had worked
答案
A
题目分析：考查时态：句意：做老师总是鼓励学生独立思考，直到他们这道数学题算了一个多小时我才告诉他们答案。主句是一般将来时，从句用一般现在时或现在完成时代替将来时，或将来完成时。选A。
点评：这题考查的是时间条件状语从句的时态问题，此类的状语从句不能用将来时，用一般现在时或现在完成时代替将来时，或将来完成时。