A massive frog drops vertically from a tree branch onto a skateboard that moves horizontally below. When the frog lands, the skateboard slows, consistent with the conservation of momentum. The impulse that slows the skateboard is a) the friction force of the frog’s feet acting backward on the skateboard x time during which the speed changes. b) equal and opposite to the impulse that brings the frog up to speed c) Both of these d) Neither of these