6.Because the wood of the dogwood tree is very hard,_____ is used for objects,such as roller skate wheels,in which hardness is desired.

(A)and

(B)it

(C)what

(D)thus

7.In hot,dry regions,the Sun's heat causes the outer layer of rocks _____,a process called exfoliation.

(A)are expanded and peeled away

(B)to expand and peel away

(C)expands and peels away

(D)they expand and peel away

8.The lower _____ in a room,the more slowly our eyes focus.

(A)the level of lighting

(B)light level

(C)leveling of light

(D)lighting is level

9._____ Sarah Orne Jewett,a nineteenth-century writer,read widely in her family's extensive library.

(A)That she received little education formally

(B)The little formal education that she received

(C)Little formal education that was received by

(D)Although she received little formal education

10.(A)In the early twentieth century,the "Model T"automobile was mass-produced and sold at a price _____ could afford.

(A)the average person who

(B)that the average person who

(C)and the average person

(D)the average person