Thousand of years ago,fire away animals and keep warm

B
be used to doing习惯于做某事，
be used to do被用来做某事
used to do过去常常做某事
这句话的意思是：几千年前，火被用来驱赶动物和取暖。

use to 用...来做某事 He uses a knife to cut.
used to do 过去常常做某事 He used to live in London.
be used to do 被用作做某事 Knives are used to cut food. 被动语态
be used to doing 习惯做某事 I'm not used to eating so much at lunch time
be used for 用来做某事；...的作用是 The knife is used for cutting.

故选B

D.
Used to doing过去常常做某事