基本语法

1. I was ill ,so I        __(不得不)stay at home today.

2. You____   (不必)return that book to me at once.

3. We        _ (可能去)to visit my father's farm during the summer vacation.

4. You    ___  (可以)call him. He   __   (不可能)do anything else this afternoon.

5. ______(或许)she will invite you to her party ,too.

上消化道大出血的临床表现中，呕血还是便血主要取决于（）

A.出血部位的高低

B.血液在胃肠内的停滞时间

C.肠蠕动的快慢

D.出血速度和量

E.凝血功能情况