Assume put options on a stock with strike prices $14 and $18 cost $2 and $4.50, respectively. The two options can be used to create a bull spread if an investor buys the __(a)____ ($14 put or $18 put) and sell the __(b)___ ($14 put or $18 put). This strategy gives rise to an initial cash ____(c)______ (inflow or outflow) of $___(d)__.