–Many people agree that _______ knowledge of English is _______ must in _______ international trade today.(本题2.0分) A、 a; /;an B、 the;a;the C、 the;a;an D、 a;a;/
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设齐次线性方程组{2x1-x2+x3=0{x1-x2-x3=0{λx1+x2+x3=0有非零解,则
设齐次线性方程组{2x1-x2+x3=0{x1-x2-x3=0{λx1+x2+x3=0有非零解,则 为( )(本题4.0分) A、 -1 B、 0 C、 1 D