Sentence Correction

Modern inventions such as the pneumatic drill may seem to have improved the work conditions
of manual laborers, but in fact, have increased the incidence of health hazards such as silicosis
because it has aggravated the dust hazard involved in excavation and demolition.

(A) have increased the incidence of health hazards such as silicosis because it has
aggravated the dust hazard
(B) has increased the incidence of health hazards such as silicosis because it has
aggravated the dust hazard
(C) have increased the incidence of health hazards such as silicosis because they
have aggravated the dust hazard
(D) increased the incidence of health hazards such as silicosis because they aggravated
the dust hazard
(E) increased the incidence of health hazards such as silicosis because of aggravating
the dust hazard

Can some one please explain me the reference of it/they here..
to whom does they refer to as per the antecedent rule, i am flummoxed whether it refers to the modern invention, pneumatic drill or manual laborers - I know this one's not obvious as per the meaning of the sentence
I am always confused in such kind of questions. Please help me with some tips.