18.   Because dinosaurs were reptiles, scientists once assumed that, like all reptiles alive today, dinosaurs were cold-blooded. The recent discovery of dinosaur fossils in the northern arctic, however, has led a number of researchers to conclude that at least some dinosaurs might have been warm-blooded. These researchers point out that only warm-blooded animals could have withstood the frigid temperatures that are characteristic of arctic winters, whereas cold-blooded animals would have frozen to death in the extreme cold.

Which one of the following, if true, weakens the researchers’ argument?

(A) Today’s reptiles are generally confined to regions of temperate or even tropical climates.

(B) The fossils show the arctic dinosaurs to have been substantially smaller than other known species of dinosaurs.

(C) The arctic dinosaur fossils were found alongside fossils of plants known for their ability to withstand extremely cold temperatures.

(D) The number of fossils found together indicates herds of dinosaurs so large that they would need to migrate to find a continual food supply.（D）

(E) Experts on prehistoric climatic conditions believe that winter temperatures in the prehistoric northern arctic were not significantly different from what they are today.

这道题为什么选D呢?不明白。。。冷不冷和寻找食物有什么关系？