题目

Progress______ so far very good and we are sure that the work will be finished on time. A. was       B. had been    C. has been    D. will be 答案：C 解析:考查了时态--现在完成时如图，在△ABC中，点D、E分别是边BC、AC的中点，过点A作AF∥BC交DE的延长线于点F，连接CF。（1）求证：四边形ABDF是平行四边形；（2）若∠CAF=45°，BC=4，CF=，求AB的长。