In Primary Mutant Screens

In primary mutant screens,

A) mutant flies are identified by morphology and then the mutant genes are isolated and studied.
B) mutant flies are created by RNAi using random RNA sequences and then the genes affected identified.
C) mutant flies are created by gain-of-function expression of random genes.
D) two mutant flies are mated to determine if there is an effect on morphology.