If r, s, and t are all positive integers, what is the remainder when 2^(rst) is divided by 10?First of all, when a positive integer is divided by 10, the remainder is the units digit of that integer.
Next, the units digit of 2 in positive integer power repeats in blocks of 4: {2, 4, 8, 6}
The units digit of 2^1 is 2;
The units digit of 2^2 is 4;
The units digit of 2^3 is 8;
The units digit of 2^4 is 6;
The units digit of 2^5 is 2, AGAIN;
...
(1) s is even --> rst is even, hence the units digit of 2^(rst) is either 4 or 6. Not sufficient.
(2) rs = 4 --> rst is a multiple of 4, hence the units digit of 2^(rst) is the same as the units digit of 2^4 so 6, which means that the remainder upon division of 2^(rst) by 10 is 6. Sufficient.
Answer: B.
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