题目

完型填空     A wise man was walking from one town to another with a few of his followers, and they happened to pass a lake. So they stopped there to     1   for a while, and the wise man said to one of his followers, “I am very   2  . Could you please    3  me some water from the lake?”     The follower walked to the lake    4   the wise man requested. Some people were washing clothes in the water, and right at that moment, several carts (手推车) started crossing the lake.    5    , the water became very muddy. Then he went back and told the wise man 在公式中，已知t．D，则d=_____________．