18) Android has a system of importance hierarchy of process； the processes are classified as: 1. Background； 2. Service； 3. Foreground； 4. Visible. Please order the above categories from "'the most important"' to "the least important".
(A) 3412
(B) 3421
(C) 4312
(D) 4123

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統計： A(0), B(0), C(1), D(0), E(0) #2926624