Suppose the observed annual quantity of steel exchanged in the European market is 30 million metric tons, and the observed market price is 90 euros per ton. If the linear demand function for steel takes the form Q = a - 0.9P, what is an appropriate value for the intercept coefficient a?
选项：

A:a = -51
B:a = 51
C:a = 111
D:a = -111