Please turn left ________ the traffic lights. [ ]
A. on
B. in
C. at
D. to
答案:C
如图,在三棱台ABC-DEF中,平面BCFE⊥平面ABC,∠ACB=90°,BE=EF=FC=1,BC=2,AC=3.
(I)求证:BF⊥平面ACFD;
(II)求直线BD与平面ACFD所成角的余弦值.
现代物流的特征可以概括为哪些?