选 C
这里是than的比较级句型,than的前后连接两个相同的成分 by air 和 by sea
it is much quicker to go by air than by sea .意思是说乘飞机比坐船快多了.
1）it 在这里做形式主语,这个句子也可以写成 To go by air is much quicker than by sea.
2）much修饰比较级quicker.再比如
Lily plays basketball much better （这里better是well副词的比较级) than Lucy does.