Let am=111…1⏟m times , then which of the following is a prime number?

Let $a_{m} = \underset{m times}{\underset{⏟}{111 \dots 1}},$ then which of the following is a prime number?

A
$a_{4}$
B
$a_{5}$
C
$a_{91}$
D
$a_{100}$

Solution:

Show that $a_{5}$ is not divisible by any prime upto $105 .$

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