如图所示为观察量筒读数时的四种方法,其中正确的是( )
A:
B:
C:
D:
用a、b、c表示三条不同的直线,α表示平面,给出下列命题:①若a∥b,b∥c,则a∥c; ②若a⊥b,b⊥c,则a⊥c;③若a∥α,b∥α,则a∥b; ④若a⊥α,b⊥α,则a∥b.
If something is _______, it is known about by only a few people.