Answer: D

Reasoning: Zinc-phosphate cement sets through an acid–base reaction between the phosphoric-acid liquid and the basic powder.  Limiting how fast the powder is incorporated (smaller, fewer increments) leaves more unreacted acid in the mix for a longer period and therefore prolongs the setting (retards the set).  In contrast, adding extra water actually accelerates the set—Windeler showed that water dilution shortened, not lengthened, the setting time of zinc-phosphate cement [Windeler, 1979, PMID 279585].