Known to

[wangming]

3. Since many casual smokers develop lung cancer and many----smokers do not, scientists believe that individuals differ in their----the cancer-causing agents known to be present in cigarette smoke.
(A) heavy.. susceptibility to
(B) chronic.. concern about
(C) habitual.. proximity to
(D) devoted.. reliance upon
(E) regular.. exposure to

In this sentence, the OA is A, and I just wanted to know what the function and meaning of "known to" in here?

[BobK]

It's a passive, concealing who's doing the knowing: 'Experts/doctors/scintists/health profeesionals know that smoking is carcinogenic.'

There's another use of 'known to', in which 'know' means 'be acquainted with or aware of the presence of' rather than 'have knowledge/understanding of': 'When you arrive at the conference centre, make yourself known to someone at the reception desk.'

(In fact, this usage is totally distinct, in that the 'known to' is followed by a noun; the first has the word 'known' followed by a to-infinitive.'

b