Suppose There Is a Competitive Market for Retail Workers at a Large

Suppose there is a competitive market for retail workers at a large shopping mall that is large enough to constitute its own labour market.The labour demand curve is Q_D = 1000 - 20(w) .The labour supply curve is Q_S = 400 + 40(w) .Suppose that a union now successfully organizes the workers at this mall and obtains a wage rate of $11 (but does not affect the demand curve) .The number of workers employed after unionization is

A) 400.
B) 780.
C) 880.
D) 1000.
E) Not determinable from the information provided.