AMC 8 Question

If n is an even positive integer, the "double factorial" notation n!! represents the product of all the even integers from 2 to n. For example, 8!! = 2x4x6x8. What is the units digit of the following sum?
2!! + 4!! + 6!! + ... +

(A)
(B)
(C)
(D)
(E)
Answer:

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