ANS:C - only one of the liquids may be pumped at any desired rate.

In a counter-current liquid extractor: only one of the liquids may be pumped at any desired rate. In a counter-current liquid extractor, the two liquid streams flow in opposite directions. The operation of a counter-current extractor relies on the principle that the solvent and the feed solution are in contact with each other for an extended period, allowing for efficient mass transfer between the two phases. Typically, one of the liquids, often the solvent, is pumped into the extractor at a controlled rate while the other liquid, usually the feed solution, flows into the extractor due to gravity or pressure. Therefore, the correct option is: only one of the liquids may be pumped at any desired rate.