To find gene(s) involved in the Drosophila Period phenotype investigators used 'forward genetics'. What does this mean?

A) The investigators knew the Period gene was involved in circadian rhythms and so used homologous recombination to knock-out the gene and found the circadian rhythm was altered.
B) The investigators thought the Period gene was involved in circadian rhythms and made a conditional knockout of the Period gene using the Cre/lox system. When the Period protein was knocked out the flies displayed a disrupted circadian rhythm.
C) The investigators used random mutagenesis to find animals with the Period phenotype and used molecular genetic methods (such as positional cloning) to identify the gene.
D) The investigators used the CRISPR/Cas9 system to selectively manipulate the Period gene and confirmed this by showing a disrupted circadian rhythm in the flies.