21) What is the pore size of most commonly used filters for separating particles and dissolved substances in marine waters?
(A) 0.45uM,
(B) 0.45mm,
(C) 0.45cm,
(D) 0.45um,
(E) 0.2mm

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統計： A(1), B(0), C(1), D(0), E(0) #2788469