90. When an iron knife is dipped in an aqueous solution of copper sulphate, a layer of reddish-brown precipitate is formed. Which compound or element is the layer of reddish-brown precipitate?
(A) iron sulphate
(B) copper
(C) rust
(D) copper sulphate
(E) iron
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統計: A(0), B(0), C(1), D(0), E(0) #3853605
統計: A(0), B(0), C(1), D(0), E(0) #3853605