？同位语从句和定语从句的区别

Jaminpei

n laboratory rats, a low dose of aspirin usually suffices to block production of thromboxane, which is a substance that promotes blood clotting, but not seriously interfering with the production of prostacyclin, which prevents clotting.

(A) which is a substance that promotes blood clotting, but not seriously interfering
(B) a substance that promotes blood clotting, but not seriously interfering
(C) a substance that promotes blood clotting, but does not seriously interfere
(D) which is a substance to promote blood clotting, but does not seriously interfere
(E) which is a substance that promotes blood clotting, but not a serious interference  

碰到一道题，解释d选项是因为but会被认为是which从句里面的，那么想问一下为什么c中的but不会被认为在从句里面
同位语从句和定语从句到底如何区分？？？！

Isabella雪

同问此题