Should we care if over 150 known species o

题目：

Should we care if over 150 known species of animals have (91) from the earth in the last fifty years Should we be concerned that there are (92) thousands of species whose very existence is (93) endangered—largely because of our activities (94) , after all, is the natural end of populations. Species are born, then (95) , and then die. Some live a (96) time, perhaps millions of years; some die more quickly. We have (97) the extinction of many species we know about, and we have undoubtedly sealed the (98) of others. In fact, there are undoubtedly many other species that have lived among us during our time on earth, but that have disappeared as a (n) (99) of our activities without our ever having known they existed.
It is hard to explain the rationale of many of us who are concerned about such matters. I have never seen a sea whale, (100) I don’t want them to become extinct. Moreover, I felt this way long before I understood anything about how they might be an important part of an ecosystem. Possibly such feelings merely reflect the cultural attitude that it is "nice" to wish other living things well; thus, the attitude is (101) . I feel nice.
There are, of course, more rational reasons for (102) the extermination of any species. For one thing, the kind of attitude that encourages or sanctions the destruction of other species is a threat (103) our own wellbeing. If such an attitude exists, we ourselves might (104) victim to it. living things (including us) might be expected to fare better where there is (105) for life. The extinction of other species could also threaten us (106) by simplifying the system of which we are a part or by destroying parts of the ecosystem (107) which we directly rely. For example, if we continue to poison the oceans (108) we are willing to believe only a few bottom dwellers are affected, we might (109) overstep some critical threshold and trigger the wholesale death of plankton, thus finding ourselves without a major (110) of the world’s food and with our oxygen supplies dwindling.

A．gain

B．enter

C．become

D．fall