题目

 --How clean the window is!    --Yes. It ________ just now.     A. has been cleaned  B. was cleaned   C. is cleaned   D. will be cleaned  答案：A观察心脏结构时，看到心脏瓣膜只能向一个方向开放，这样保证了血流方向是由A.心房——心室——静脉B.心房——心室——动脉C.心室——心房——动脉D.静脉——心室——心房