People traveling long distances frequently have to decide ______they would prefer to go by land, sea or air.

A．which

B．how

C．why

D．whether

炉渣垫层采用（）的拌和料铺设，其厚度不应小于80mm

A.石灰、砂（可掺入少量黏土）与碎砖

B.炉渣或水泥与炉渣或水泥、石灰与炉渣

C.熟化石灰与黏土（或粉质黏土、粉土）

D.水泥砂浆与水泥混凝土