21.如圖所示電路，已知：I 1 = 4 A，I 2 = 6A，I 3 = -1 A，I 4 = 3 A，則I 5 = ？ 
(A)-8 A
(B)2 A
(C)4 A
(D)-9 A

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統計： A(11), B(15), C(77), D(9), E(0) #929925