Portfolio B contains a European put option and a share. Compare this with the alternative of cash, currently worth ke^-r(T-t)-. At time T Portfolio B will be worth at least as much as the cash alternative. Why must Portfolio B be worth at least as much as the cash alternative?

What is the lower bound for a 3-month European put option on Share X if the share price is 95, the exercise price 100 and the risk-free rate 12% pa?