撒哈拉以南的非洲，人种以白种人为主。[ ]
射线胶片中的溴化银微粒银的正离子失去电子，使曝光的胶片中形成所谓的“（）”。
The weather in Beijing is colder than ______. [ ]
A. in Hefei
B. Hefei
C. that in Hefei
D. that of Hefei
