A survey of 500 farmers showed that of the​ farmers, 129 grew only​ wheat, 112 grew only​ corn, 91 grew only​ oats, 199 grew​ wheat, 58 grew wheat and​ corn, 49 grew wheat and​ oats, and 178 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.