Analysts blamed May's sluggish retail sales on unexciting merchandise as well as the weather, colder and wetter than was usual in some regions, which slowed sales of barbecue grills and lawn furniture.

(A) colder and wetter than was usual in some regions, which slowed
(B) which was colder and wetter than usual in some regions, slowing
(C) since it was colder and wetter than usually in some regions, which slowed
(D) being colder and wetter than usually in some regions, slowing
(E) having been colder and wetter than was usual in some regions and slowed

please,explain why which is not needed in right answer. OA is B