A survey of 500 farmers showed that of the​ farmers, 126 grew only​ wheat, 106 grew only​ corn, 87 grew only​ oats, 206 grew​ wheat, 65 grew wheat and​ corn, 51 grew wheat and​ oats, and 182 grew corn. Determine the number of farmers who ​a) grew at least one of the​ three, ​b) grew all​ three, ​c) did not grow any of the​ three, and ​d) grew exactly two of the three.