你好.
He had more dictionaries than____B_____ for his work.
== He had more dictionaries than THEY were necessary for his work.
这里的they 指的是the dictionaries（必要的字典）,在有了明确的上下文的情况下,they被省略了.如果选项A中的are变成were,这个选项就对了.又如：
She has got less money than ( it ) is good for the handbag,so she cannot buy it.
括号中的 it 指的是the money ------必要的钱
更简单但也是正确的表达是：
1 He had more dictionaries than NECESSARY for his work.
2 She has got less money than GOOD for the handbag,so she cannot buy it.
---------连系动词were或is也被省略了.