单项选择题
Had electronic computers not been invented, many problems of space flight（）now.
A. could not have been solved B. could not be solved C. must not be solved D. should not have been solved
单项选择题 There has been a decrease（）the number of students entering the Medical College this year.
单项选择题 March is the third month of（）
单项选择题 One of the serious problems facing us is（）of how to get enough manpower,
