In Drosophila, the sex is determined by

1. whether the egg is fertilized or develops parthenogenetically
2. the ratio of number of X-chromosomes to the sets of autosomes
3. X and Y chromosomes
4. the ratio of pairs of X-chromosomes to the pairs of autosomes

Calvin Bridges demonstrated that in Drosophila, the sex is determined by the ratio of the number of X chromosomes to the sets of autosomes.