Indicate which of the following represent a variable and which a constant:
a. The number of letters in the alphabet
b. The number of hours in a day
c. The time at which you eat dinner
d. The number of students who major in psychology at your university each year
e. The number of centimeters in a meter
f. The amount of sleep you get each night
g. The amount you weigh
h. The volume of a liter