1. If $(3 x+1)^{5}=a x^{5}+b x^{4}+c x^{3}+d x^{2}$ $+e x+f$, then the value of $a-b \div c-d+e-f$ is ( ).
(A) -32 .
(B) 32 .
(C) 1024 .
(D) -1024 .
B

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