11. A significant proportions of the plants and animals of Hawaii exists nowhere
a b c
else in the world.
12. You will not be able to pass your examination except you study harder than you
a b c
解析： 表达“除非”的意思；except 改为unless。
13. Some bosses dislike to allow people to share their responsibilities; they keep
all important matters tightly in their own hands.
解析： dislike doing sth. 指“不喜欢做…”。
14. Generally speaking, the bird flying across our path is observed, and the one
staying on the tree near at hand is passed by without noticed.
解析： without是介词，后面应该接名词或动名词，同时，该句又是被动的含义所以应改为: without being noticed.
15. On the whole, ambitious students are much likely to succeed in their studies
than those with little ambition.
解析： 此句是一个包含了比较级的句子，所以B项应改为： are more likely.