Let the positive numbers $a, b, c, d$ be in A.P. then $a b c, a b d, a c d, b c d$, are\

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(d) : As $a, b, c, d$ are in A.P.\ $\Rightarrow d, c, b, a$ are in A.P. $\Rightarrow \frac{1}{d}, \frac{1}{c}, \frac{1}{b}, \frac{1}{a}$ are in\ H.P. H.P.\ $\Rightarrow \frac{a b c d}{d}, \frac{a b c d}{c}, \frac{a b c d}{b}, \frac{a b c d}{a}$ are in H.P\ $\Rightarrow a b c, a b d, a c d, b c d$ are in H.P.\