单项选择题

Working alone，a small pump takes twice as long as a large pump takes to fill an empty tank．Working together at their respective constant rates，the pumps can fill the tank in 6 hours．How many hours would it take the small pump to fill the tank working alone（）

A. 8
B. 9
C. 12
D. 15
E. 18

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单项选择题
A research scientist wants to study a certain attribute of dogs． It is estimated that approximately 5 percent ofall dogs have this attribute．If the scientist wants to study a sample of N dogs having the attribute，approximately how many dogs should be screened in order to abtain the desired sample size（）

A.
B. 5N
C. 20N
D. 105N
E. 120N
单项选择题
The cost，in dollars，for an appliancerepair at a certain company is 1.2p+20h，where p is the wholesale price of the parts，in dollars，and，2 is the number of hours it takes to repair the appliance．What is the cost of repairing an appliance if the wholesale price of the parts is $15and it takes 2 hours to repair it？（）

A. $12
B. $18
C. $20
D. $40
E. $58
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