Answer: C

By units digits it means the last right digit,

We assume x is 1,2,3,….., n as it is said that it’s a positive integer

x = 1 A = 6^1 = 6 B=4^2*1=(16)

x = 2 A = 6^2 = 36 B=4^2*2= (16)^2

x = 1 A = 6^3 = 216 B=4^2*3= (16)^3

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x = n A = 6^n B=4^2*n= (16)^n

So they both end in 6. And the answer is C.

Because both 6 and 16 end in 6, when they have any positive integer power, they will end with 6,

definitely this rule is not true for all other numbers. For instance 2 in different powers might end in 2,4,8.
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