Over time, housing shortages caused by rent control
选项：

A:a. increase, because the demand for, and supply of, housing are less elastic in the long run.;
B:b. increase, because the demand for, and supply of, housing are more elastic in the long run.;
C:c. decrease, because the demand for, and supply of, housing are less elastic in the long run.;
D:d. decrease, because the demand for, and supply of, housing are more elastic in the long run.