If there were no disasters in the world,life---much easier.A.will be B.would have been C.could have

If there were no disasters in the world,life (would be) much easier.
D. would be 正确；

因为这是祈使句标准组合，前面虽用的 were ，和事件无关。后面基本都跟着用 would be.

如果句子前面用的是过去完成时，后面一般就也用 would/could/might + 完成式
<例>
If you had gone to that party, we (would/could/might) have had a much better time.

那就是属于假设语气的范畴了，事情已经发生（或没发生）无法改变。

if子句用於表示几乎不可能或完全不可能的状况时, 主要子句中的助动词要用过去时表示.
would是will的过去式.

If+主词+过去简单式....主词+could/might/should/would+动词
If+主词+were/wals....主词+could/might/should/would+动词
例: If I had the money, I would set up an organization to help tsunami victims.
如果我有钱的话,我会成立一个机构来帮助海啸受难者.
(实际上, 我现在并不有钱.)

would be tend to desire something ,like you said disater it existed but people hope can avoid .

could be 是本来可以的意思，如果没有灾难，生活可以更简单，本来是可以的，因为有了灾难就不可以了，灾难是不可能没有的，不能用would be

这个问题很简单，是高中生学习的虚拟语气，你找相关的语法书一看便知。前句是对现在情况的虚拟，所以主句也应该是对现在的情况虚拟。如果选B，那么If从句就应该是if there had been no disasters in the word.虚拟语气的从句和主句要注意时态的一致性