Imagine you regressed earnings of individuals on a constant, a binary variable ("Male") which takes on the value 1 for males and is 0 otherwise, and another binary variable ("Female") which takes on the value 1 for females and is 0 otherwise. Because females typically earn less than males, you would expect:
a. none of the OLS estimators to exist because there is perfect multicollinearity.

b. the coefficient for Male to have a positive sign, and for Female a negative sign.

c. this to yield a difference in means statistic.

d. both coefficients to be the same distance from the constant, one above and the other below.