frequently的用法属于正常用法，在此修饰系动词 are。在此语境中，也可以放在句首或句末，意思不变，但过于强调，没有必要。句意：
A compound noun is a fixed expression which is made up of more than one word and functions as a noun.Such expressions are frequently combinations of two nouns.
复合名词是由一个以上的词组成的固定表达，起名词的作用。这种表达通常是两个名词的组合。
