05完形19题，不理解，请指教

为了大家更清楚，把原题弄了上来。

Section Ⅰ Use of English
Directions:Read the following text. Choose the best word(s) for each numbered blank and mark A,B,C or D on ANSWER SHEET 1(10 points)

The human nose is an underrated tool. Humans are often thought to be insensitive smellers compared with animals, 1 this is largely because, 2 animals ,we stand upright. This means that our noses are 3 to perceiving those smells which float through the air, 4 the majority of smells which stick to surfaces. In fact 5 , we are extremely sensitive to smells, 6 we do not generally realize it. Our noses are capable of 7 human smells even when these are 8 to far below one part in one million.

Strangely, some people find that they can smell one type of flower but not another, 9 others are sensitive to the smells of both flowers. This may be because some people do not have the genes necessary to generate 10 smell receptors in the nose. These receptors are the cells which sense smells and send 11 to the brain. However, it has been found that even people insensitive to a certain smell 12 can suddenly become sensitive to it when 13 to it often enough.

The explanation for insensitivity to smell seems to be that brain finds it 14 to keep all smell receptors working all the time but can 15 new receptors if necessary. This may 16 explain why we are not usually sensitive to our own smells we simply do not need to be. We are not 17 of the usual smell of our own house but we 18 new smells when we visit someone else’s. The brain finds it best to keep smell receptors 19 for unfamiliar and emergency signals 20 the smell of smoke, which might indicate the danger of fire.
1. ［A］although ［B］as ［C］but ［D］while
2. ［A］above ［B］unlike ［C］excluding ［D］besides
3. ［A］limited ［B］committed ［C］dedicated ［D］confined
4. ［A］catching ［B］ignoring ［C］missing ［D］tracking
5. ［A］anyway ［B］though ［C］instead ［D］therefore
6. ［A］even if ［B］if only ［C］only if ［D］as if
7. ［A］distinguishing［B］discovering ［C］determining［D］detecting
8. ［A］diluted ［B］dissolved ［C］determining［D］diffused
9. ［A］when ［B］since ［C］for ［D］whereas
10. ［A］unusual ［B］particular ［C］unique ［D］typical
11. ［A］signs ［B］stimuli ［C］messages ［D］impulses
12. ［A］at first ［B］at all ［C］at large ［D］at times
13. ［A］subjected ［B］left ［C］drawn ［D］exposed
14. ［A］ineffective ［B］incompetent ［C］inefficient［D］insufficient
15. ［A］introduce ［B］summon ［C］trigger ［D］create
16. ［A］still ［B］also ［C］otherwise ［D］nevertheless
17. ［A］sure ［B］sick ［C］aware ［D］tired
18. ［A］tolerate ［B］repel ［C］neglect ［D］notice
19. ［A］availabe ［B］reliable［C］identifiable［D］suitable
20. ［A］similar to［B］such as ［C］along with ［D］aside from

The brain finds it best to keep smell receptors 19 for unfamiliar and emergency signals 20 the smell of smoke, which might indicate the danger of fire.
我的问题是，19为什么不能选C？（答案是A：availabe）

The brain finds it best to keep smell receptors availabe for unfamiliar and emergency signals  such as the smell of smoke, which might indicate the danger of fire.
划线部分应该是宾语。蓝色部分， it后面是不是少了is？如果不是，怎么理解呢？

谢谢啊 顶 一定要取得胜利