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1. 3 has cyclicity of 4, with last digits 3,9,7,1. Powers of 5 can only yield 5 as unit digit. So, this relation is true only for n = k =0. So, n = 0.
So, \(2^x\) = 1 , only when x =0, i.e. m =0. SUFFICIENT

2. Holds true for, n=0, m =0 and m=1/3 , n=1.. INSUFFICIENT

Answer A
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Guys,
While solutions have already been posted for this question, please make sure to take a note of this.

Statement 1: 3^5n=5^3k, where k is an integer.
This case is possible only when n=k=0. Why? Try to analyse the cycles of 3 & 5.
3^x, where x=1,2,3,4,5,6,7... is.. 3,9,27,81,243,729, 2187, 6561.. so you can see that the unit digit cycles after every 4 powers of 3 and follows a pattern, i.e, 3,9,7,1,3,9,7,1,3... Whereas,
5^y, where y=1,2,3.. is.. 5,25,625.. it always ends with 5.
Now try to analyse the fact that no power of 3 ends with 5 (power of 3 cycles within 3,9,7,1). Hence the only way they can be same is when both the powers are 0 and 3^0=5^0 = 1.
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