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Q.

Which of the following is a prime number?
 


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a

81

b

87

c

91

d

97 

answer is D.

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Detailed Solution

We have to find which one is the prime number.
A number is a prime number when it has only 2 factors i.e., 1 and the number itself.
We can write 81 as -
81=3×27  , which means it has more than 2 factors, so 81 is not prime.
We can write 87 as –
87=3×29  , which means it has more than 2 factors, so 87 is not prime.
We can write 91 as –
91=7×13  , which means it has more than 2 factors, so 91 is not prime.
But we cannot write 97 as the product of 2 numbers other than 1 and 97.
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