单项选择题A．The five questions were not well designed.B．Not all the questionnaires were returned.C．Only a small number of students were surveyed.D．Some of the answers to the questionnaire were not valid.

单项选择题A．They spent three hundred dollars on their vacation.B．They drew money than they should have from the bank.C．They lost their bankbook.D．They had only three hundred dollars in the bank.