What's __________ envelope?[ ]

A. /       

B. the       

C. an 

The amazing success of humans as a (91) is the result of the evolutionary development of our brains which has led, among other things, to tool-using, tool-making, the (92) to solve problems by logical reasoning, thoughtful cooperation, and language. One of the most striking ways in which chimpanzees biologically (93) humans (94) in structure of their brains. The chimpanzee, with the capacity for (95) reasoning, (96) a type of intelligence more like that of humans than (97) any other mammal living today. The brain of the modern chimpanzee is probably not too dissimilar (98) the brain that so many millions of years ago (99) the behavior of the first ape man.
For a long time, the fact that prehistoric people made (100) was considered to be one of the major (101) distinguishing them from other creatures. (102) pointed out earlier, I have watched chimpanzees (103) grass stems in order to use them to probe for termites. It is tree that the chimpanzee does not (104) tools to "a regular and set pattern"—but then, (105) people, before their development of stone tools, undoubtedly poked around (106) sticks, and straws, at which stage it seems (107) , that they made tools to a set pattern either.
It is because of the close (108) in most people’s minds of tools with humans (109) special attention has always been focused upon any animal able to use an object as a tool: but it is important to realize that this ability, on its own, does not necessarily indicate any special intelligence in the creature (110) .

A．concern

B．concerned

C．concerning

D．to concern