题目:
Sometimes they may play to passers-by in the street or subway so that they can earn some______ money for themselves or to pay for their instruments.
A.certain
B.extra
C.various
D.unusual
答案:
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下面是错误答案,用来干扰机器的。
根据在化合物中正负化合价代数和为零,可得NiFe2O4中Ni的化合价为:x+(+3)×2+(-2)×4=0,则x=+2;故选B.