A well stirred tank initially contains $750$ kg$,25\%$ by mass of sugar solution. The tank is fed with $5$ kg$/$ min of sugar solution having a concentration of $5\%$ by mass. At the same time, $2,5$ L$/$min of solution discharges from the tanks.

a$)$ Assuming constant density, which of the following differential equations describe the variation CA $($mass fraction of sugar$)$ with time t $($minutes$).$

b$)$What will be the sugar concentration $(\%)$ after $20$ min?

c$)$ What will be the kg of sugar when the tank contains $1000$ kg solution?