Answer: A

Reasoning: In alginate impression materials, calcium sulfate reacts with sodium alginate to form the gel, while trisodium (or other sodium) phosphates are added to tie-up some of the calcium ions temporarily and thus slow the setting reaction. By delaying the availability of free calcium, trisodium phosphate lengthens the working and setting times—i.e., it serves as a retarder [Jarvis & Earnshaw, 1981, PMID 7018476; Lemon et al., 2003, PMID 12942062].