24. In time `t`, an amount of heat `Q` flows through the solid door of area `A` and thickness `d` represented above. The temperatures on each side of the door are `T2` and `T1`, respectively. Which of the following changes would be certain to decrease `Q` ?
(A) Increasing `A` only
(B) Decreasing `d` only
(C) Increasing `d` and `T2−T1` only
(D) Decreasing `A` and `T2−T1` only