Which of the following best describes a first-order differential equation that allows the variables to be isolated on opposite sides of the equation?
a) It can be rewritten in the form dxdy+P(x)y=Q(x), where P and Q involve only x.
b) It can be rewritten in the form g(y)dxdy=f(x), where f involves only x and g involves only y.
c) It can be rewritten in the form dxdy=f(x)+g(y), where f involves only x and g involves only y.
d) It involves y but contains no explicit appearance of x on its right–hand side.