1. As it becomes more frequent to have spouses who both work outside the home, companies are beginning to help in finding new employment for the spouses of transferred employees.

(A) it becomes more frequent to have spouses who both work outside the home

(B) it becomes more frequent to have couples both working outside the home

(C) it becomes more common that both husband and wife should be working outside the home

(D) it becomes more common for both husband and wife to work outside the home（D）

(E) couples in which both of the spouses working outside the home become more common

请问这里it指代什么呢？