In problems below, (a) identify the independent variable and the dependent variable of each equation (use 't' for the independent variable if an independent variable is not given explicitly); (b) give the order of each differential equation (enter '1' for first order, '2' for second order and so on; do not include the quotes); and (c) state whether the equation is linear or nonlinear. if your answer to (c) is nonlinear, make sure that you can explain why this is true.