題目：某地區棲息著一種蝸牛,不同個體具有不同的顏色,但科學家發現此區樹幹上的此種蝸牛多為深咖啡色殼，而草地上多為淺黃綠色殼。若以天擇說解釋此現象，下列何者最為合理？                                                                                                                                                              A：在樹幹上的蝸牛容易突變成深咖啡色殼                                                                                                                                                          B ：兩處蝸牛殼色明顯不同是受天敵影響的結果                                                                                                                                                          C :蝸牛個體的殼色是由出生時環境的顏色決定                                                                                                                                                          D :不同殼色的蝸牛在草地上被捕食的機率相等                                                                                                                                                                       答案是B，但為什麼不是A或 C