In the past few decades the wages of high-skill workers increased relative to those of low-skill workers. Under what circumstances can we say that this development is due to an increase in the relative demand for high-skill labour?

a. There was a simultaneous increase in the relative supply of high-skill workers

b. There was a simultaneous increase in the relative employment of high-skill workers

c. There was a simultaneous increase in the relative employment of low-skill workers

d. There was a simultaneous increase in the relative supply of low-skill workers