Why didn't Mendel discover the nature of sex chromosomes in his pea plants?

A)Peas, and most other plants, do not have sex chromosomes.
B)He focused only on the flowers and the seeds.
C)The 3:1 and 9:3:3:1 ratios of offspring did not fit the pattern of sex determination in peas.
D)The sex chromosomes were too difficult to see with his rudimentary microscope.