Which of the following represents a synthesis reaction?

ac + b → ab + c
a + b + c → abc
abc → a + b + c
ab + cd → ac + bd

If it is an isoelectronic series select true, if not select false. o2-, s2-, se2-, te2- na+, k+, rb+, cs+ n3-, p3-, as3-, sb3- ag, cd+, sn3+, sb4+ f-, cl-, br-, i- f-, ne, na+, mg2+ s2-, s, s6+