单选题 18. The pepper Capsicum chinense grows as a native wild plant only in South America. Birds swallow the pepper's fruit whole, thereby providing the plant with its means of distributing seeds. Domesticated varieties of C.chinense grow in the Caribbean islands as well as in South America. Although it is unknown just when C.chinense was introduced into the islands, the introduction of the species was probably the result of human activity, because ______.
Which of the following most logically completes the argument? ______

A、 the fruits of the domesticated varieties are too large for birds to swallow
B.
B、 chinense grows in the Caribbean islands under climatic conditions very similar to those under which the wild variety grows
C、 the peppers are an important part of the Caribbean islands' characteristic cooking
D、 most of the plant species in the Caribbean islands originated is South America
E、 many of the species of birds that are found in the Caribbean islands spend only the winter there

【正确答案】 A

【答案解析】 A：因为鸟类吞不下这些果实，所以不可能是鸟类传播的，这加强了人类传播的可能性。正确。
B：无关选项。
C：无关选项。
D：不能解释为什么是人类引入的。
E：无论在哪里过冬，鸟类依然有可能传播种子，不能解释文章。