what...is/are
 【问】 作“the thing which”解的what用作主语时，其谓语动词可以用复数吗?
 【答】 一般都把what理解为“the thing which”，即理解为单数，因此，what在从句中充当主语时，其谓语动词应取单数形式：
 1. Give me what there is.(Palmer)
 把有的(东西)都给我。
 2. What is over is over.(Wilde)
 过去的事情就让它过去吧。
 3. What was once a palace is now a pile of rubble.(CGEL)
 过去曾经是宫殿的地方现在是一片瓦砾。
 但是，有时候人们把what理解为复数(those which，或the things which)：
 1. We can see what appear to be camels.(Wood)
 我们能看到的似乎是骆驼。
 2. What were once truth are true no longer.
 曾经是真理的东西现在已不再是真理了。
 3. In some countries，what are called“Public Schools” are not owned by the state.
 在某些国家，被叫做“公立学校”的机构并不归国家所有。
 当what从句充当主语从句时，其谓语动词的数也是取决于对what所代表的数的理解：
 1. What I should like most for a birthday present is a camera.(Wood)
 我最喜欢的生日礼物，就是一个照相机。
 2. What were supposed to be new proposals were in fact modifications of earlier ones.
 被看作是新建议的东西，其实是先前建议的改头换面而已。
 3. You need not get any more stamps;what we have are quite sufficient.(Wood)
 你不必再买邮票了;我们手头上的已足够了。
 4.
 我所说的和我所做的是我自己的事。
 5. What are sovereign property rights to some people are nothing at all to Stanley.(Crews)
 在某些人看来是至高无上的财产权，对斯坦利来说是毫无意义的。
 6. What were left behind were five empty bottles.
 所留下的东西是五个空瓶子。
 7. What the bourgeoisie，therefore，produces，above all，are its own gravediggers.
 它首先生产的是它自身的掘墓人。(《共产党宣言》，P.35)
 请做下面的英语试题：
 One of the most difficult questions in defining sleep is “what is the functions of sleep?” (TOEFL，91. 8)