Health experts believe that even a little exercise is better than______ at all.

A．none

B．little

C．no one

D．few

设α₁，α₂，α₃，α₄线性无关，则（）线性无关。

A.α₁+α₂，α₂+α₃，α₃+α₄，α₄+α₁

B.α₁+α₂，α₂+α₃，α₃+α₄，α₃-α₄

C.α₁，α₁+α₂，α₁+α₂+α₃，α₁+α₂+α₃+α₄，α₄+α₁

D.α₁-α₂，α₂-α₃，α₃-α₄，α₄-α₁