34. In plant vascular system, what primary characteristic enables phloem sap to move bidirectionally?
(A) Positive pressure gradient
(B) Living sieve-tube elements
(C) Proton pump mechanisms
(D) Symplastic communication
(E) Companion cell interactions
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統計: A(5), B(1), C(1), D(1), E(1) #3429713
統計: A(5), B(1), C(1), D(1), E(1) #3429713