21. Several comet are discovered each year, but very few of them are bright enough to be seen without the aid of magnification.
这里错误是comet应该是复数,但是to be seen错了没有?是不是改为to see
32. The black leopard is very dark that its spots are difficult to see.
这里的错误答案是very改为so,组成so that。可这里to see用的主动态,没用被动。