Helen was fully occupied_____business matters, so we didn’t want to brother her.

A．for

B．at

C．in

D．with

设n维向量组α₁，α₂，…，α_s(s＜n)线性无关，则β₁，β₂，…，β_s线性无关的充分必要条件是
(A) α₁，α₂，…，α_s可由β₁，β₂，…，β_s线性表出．
(B) β₁，β₂，…，β_s可由α₁，α₂，…，α_s线性表出．
(C) α₁，α₂，…，α_s与β₁，β₂，…，β_s等价．
(D) 矩阵A=(α₁，α₂，…，α_s)与矩阵B=(β₁，β₂，…，β_s)等价．