由果到因的研究方法是
A．现况调查B．病例对照调查C．队列调查D．临床试验E．社区试验
参考答案：B
I have been studying computer for years and I still ___.
A．have
B．do
C．have been
D．am
以下循环的执______。 k=0 Do Until k＞=10k=k+1 Loop
