Technology is another great force for change. In part technology has caused the population explo-sion. Many of us won't be alive if it weren't for advances in health, agriculture and industry.
解题思路：本句...if it weren't for advances in health,agriculture and...是一个虚拟条件句，谓语动词要用虚拟语气，所以won't是错的，应改为wouldn't。例如：If it were not for the expense involved,I would go there by air.如果不是因为花费大，我就坐飞机去那里了。If the shop had packed the goods properly,they wouldn't have been damaged.如果商店装好了，这些商品就不至于损坏。答对本题的考生只有31%。有些考生误把weren't改为wasn't。在虚拟条件句中，if后面可以用were或weren't,因此不必改为was或wasn't。还有些考生把weren't改为isn't...many of us won't...if it isn't...这样的句子语法结构是对的，但这是一种真实条件句。本句意思是：“如果没有医疗、农业和工业方面的成就，我们中许多人可能现在已不在人世”。显然，这是一种与事实相反的假设的条件，本句必须要用虚拟语气。有些考生把advances改为advance。advance作名词表示“进展、成就”时，可以用复数。所以，本句中用advances是对的。
Now he had the capability to leave that planet and move out into the universe to those worlds he has known previously only indirectly .
解题思路：此题是主从句时态使用不一致。从此题在句子中的谓语动词、时间状语（now)及全篇的上下文来看，应该用一般现在时has。此题考生需要注意的是不要受前后句中的现在完成时时态的影响而改成has had的形式。因为动词have在表示“拥有”含义时，通常只用动词一般式，而不用完成时。例如：I have one brother.(不说I have had one brother.)