If \( a, b, c, \) and \( d \) are integers and \( ab^2c^3d^4 > 0 \), which of the following must be positive?
I. \( a^2cd \)
II. \( bc^4d \)
III. \( a^3c^3d^2 \)
View explanation
Correct answer: C
I. \( a^2cd \). \( a^2 > 0 \). \( cd \)? \( c, d \) signs unknown. Uncertain.
II. \( bc^4d \). \( c^4 > 0 \). \( bd \)? Unknown. Uncertain.
III. \( a^3c^3d^2 = (ac)^3 d^2 \). \( ac > 0 \implies (ac)^3 > 0 \). \( d^2 > 0 \). Product positive. True.
The correct answer is C (III only).