Correct answer: (B) Ultrafiltration is used as pretreatment for RO.; (C) Membrane fouling occurs in microfiltration and RO.
Explanation
Ultrafiltration removes particles and colloids that would foul a downstream RO membrane, so B is true. Both microfiltration and RO can foul, so C is true. The usual pretreatment order runs from coarser separation toward RO, not the reverse in A. Solute accumulation at the RO membrane surface causes concentration polarization, making D false. Select **B, C**.
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