[問題]考古題form17-Q15

[sweetymanman]

The breathing capacity of nonsmokers exposed to tobacco smoke on the job has been found to be significantly less than nonsmokers working in smoke-free environments, although unrestricted smoking at work is permitted by nearly 75 percent of all employers in the United States.

A)The breathing capacity of nonsmokers exposed to tobacco smoke on the job has been found to be significantly less than nonsmokers working in smoke-free environments, although

B)The breathing capacity of nonsmokers exposed to tobacco smoke on the job has been found significantly less than nonsmokers working in smoke-free environments, however

C)Although the breathing capacity of nonsmokers exposed to tobacco smoke on the job has been found significantly less than that of nonsmokers working in smoke-free environments, however

D)Although the breathing capacity of nonsmokers exposed to tobacco smoke on the job has been found to be significantly less than that of nonsmokers working in smoke-free environments,

E)Although the breathing capacity of nonsmokers exposed to tobacco smoke on the job has been found to be significantly less than nonsmokers working in smoke-free environments,

KEY:D
這一題答案我覺得是OK的，但有一個地方小小不明白，想和大家討論

正確選項（D）中的has found to be ，to be是什麼意思呢？
一定要用嗎？還是可以省略？