For each of the following compounds, indicate the aldehyde or ketone from which it would be formed by an aldol addition:
a. 2-ethyl-3-hydroxyhexanal
b. 4-hydroxy-4-methyl-2-pentanone
c. 2,4 -dicyclohexyl- 3 -hydroxybutanal
d. 5 -ethyl- 5 -hydroxy- 4 -methyl- 3 -heptanone
