1. I have made things clear. 如果选D,这句话到这里就完整了. 但是你后面有that从句, 所以用it, I have made it clear it指什么呢? it 指that后面的从句, nobody is allowed to smoke here. 也就是说 I have made "Nobody is allowed to smoke here" 这件事情 clear.
2. 这个跟上面几乎正好是反过来的例子. She was glad that her success would make things easier, 到这里就完整了, 后面for the women who would follow, 有没有, 对句子成分没有影响. 如果换成it, She was glad that her success would make it easier for the women who would follow, it指什么呢?
其实多读读, 掌握语感最重要.