How many iterations will the following for loops execute?

a. for (int i = 0; i < 20; i++) { }

b. for (int i = 1; i <= 20; i++) { }

c. for (int i = 5; i < 20; i++) { }

d. for (int i = 20; i > 0; i--) { }

e. for (int i = 1; i < 20; i = i + 2) { }

f. for (int i = 1; i < 20; i *= 2) { }