A、The vomeronasal system. B、The metabolism system. C、The digestive system. D、The respiratory system. A此题也属于常识问题，如果考生不清楚的话也可以从文章中

Picture this: A foraging squirrel comes into range of a hungry, coiled rattlesnake. The squirrel realizes the danger too late -- the rattlesnake strikes out and bites, fangs pumping venom deep into the squirrel’s small body. The poison doesn’t kill immediately, but the squirrel’s death war- rant is signed all the same. Even as he squirms away from the snake and bounds to his hiding place in the underbrush, the venom works its way through his system, dissolving his tissues and eventually killing him. Meanwhile the snake slithers after the departed squirrel, taking his time. Uncannily, he homes right in on the squirrel’s hiding place. How did the snake know where his prey was hiding? Do snakes have some kind of magical sixth sense? Actually, snakes DO have a kind sixth sense, but it has nothing to do with magic.
Although snakes often have very sharp vision, a sense of smell, and a sensitivity to heat and vibrations, their most important sensory system is one that we humans don’t share with them at all. This sixth sense is called the "vomeronasal system", and the sense organs for this are two tiny, bulb-shaped structures located on the roof of the snake’s mouth. This sensory system is somewhat similar to smell -- in that it senses the presence of chemicals in the snake’s environment. Snakes can use this extra sense to detect the faint chemical traces left by departing prey, or they can use it to track down a mate. On our next program we’ll learn how snakes use their flicking, forked tongue to aid this sixth sense in the quest for prey.