Draw the two chair conformers for each of the following and indicate which conformer is more stable:
a. cis-1-ethyl-3-methylcyclohexane
b. trans-1-ethyl-2-methylcyclohexane
c. trans-1-ethyl-2-isopropylcyclohexane
d. cis-1,2-diethylcyclohexane
e. cis-1-ethyl-3-isopropylcyclohexane
f. cis-1-ethyl-4-isopropylcyclohexane