题目

 Believe it or not, this is the first time that I _____ the film So Young. A. saw                 B. have seen              C. had seen         D. will see 答案：B在同一直线上有三个点A，B，C，若AB=10cm，AC=30cm，点M是AB的中点，点N是AC的中点，求线段MN的长．