单项选择题
Scientists say it may be five or six years （） this medicine is tested on human beings.
A．since B．before C．after D．when
单项选择题 （） the obvious differences in size and population, the states of America have many things （） common.（）
单项选择题 A club is a place to make frequent（） with friends.
单项选择题 As is well known, eating too much fat can （） heart disease and cause high blood pressure.
