现在完成时和过去时态的区别

题目

现在完成时和过去时态的区别
The monent the 28th OLympic Games________open,the whole world cheered.
A.declared B.have been declared C.have declared D.were declared
为什么选D不选B?
Since the deadly H5N1 bird flu virus_____not only in Asia but also in Europe and Africa,migratory birds have been regarded as the carriers.
A.discovered B.was discovered C.had discovered D.have been discovered
这题又为什么选B不选D啊?不是有个SINCE啊?

答案

1.28届已经是过去了 所以用 D.were declared
2.discover是指发现 D指的是过去到现在一直发现表示状态 题意是发现这个时刻之类的