Many Receptor Tyrosine Kinase Genes Are Essential, Making It Difficult

Many receptor tyrosine kinase genes are essential, making it difficult to assess the function of such genes. How did researchers get around this problem when examining the roles of the sev gene?

A) The researchers looked at mutations in related genes instead of mutations in the sev gene
B) The researchers used sub-lethal mutations in the sev gene
C) Haploid flies were used for experiments
D) The researchers used conditional mutants