Fill in the blanks to make the following statements correct.
a. If all workers were identical, all jobs had identical working conditions, and labour markets were perfectly competitive, then all wages would be _______.
b. In reality there is wide variation in wages. Four reasons for wage differentials are _______, _______, _______, and _______.
c. A country that invests in its public education and public health systems is improving the _______ of its population.
d. If there are two groups of workers in a competitive labour market, and one group becomes discriminated against, the labour supply curve for jobs that remain available for that group will shift to the _______ and the wages for these jobs will _______.