GMATPREP3 SC- In an effort to reduce the number of fires

In an effort to reduce the number of fires started by cigarettes, a major tobacco company is test-marketing a cigarette in which thin layers of extra paper are used to decrease the amount of oxygen entering the cigarette, thereby slowing the rate at which it burns and lowering the heat it generates.

A.in which thin layers of extra paper are used to decrease the amount of oxygen entering the cigarette, thereby slowing
No error.

B. in which they use thin layers of extra paper in decreasing the amount of oxygen entering the cigarette, which slows
- "they" cannot be used to replace "a company"
- "in decreasing" is the wrong usage. This implies that the company is decreasing oxygen entry in some other way, but in the process of decreasing, it's using these paper layers. We want to say that they use these layers of paper IN ORDER TO decrease oxygen.

C. that uses thin layers of extra paper to decrease the amount of oxygen when it enters the cigarette, thereby slowing
- "a cigarette that uses" implies that the cigarette itself is actively using something. For inanimate objects, the GMAT will usually prefer to use a construction such as "in which," in order to "de-personify" the object. (Note: the GMAT is not always totally consistent on this).
- ambiguous pronoun "it." In the structure "a cigarette that uses ____ when it ____," the "it" should refer to "cigarette." That would be nonsensical here.

D. for which thin layers of extra paper are used to decrease the amount of oxygen that enters the cigarette, and thereby slowing
- the issue is "and." If "and" were omitted, "thereby slowing" would be correctly modifying the previous clause. The "and" suggests that what comes after should be another verb, clause, or parallel modifier: e.g. "... and which thereby slows..."

E. using thin layers of extra paper in decreasing the amount of oxygen entering the cigarette, which slows
- same issue with implied active "using" and in C.
- "in decreasing" - same as B
- a dependent clause beginning with "which" should refer to the noun directly before it. This should say "slowing" instead.

Does that answer your questions?