Daisy received a tape recorder as a birthday gift and is not able to return it. Her utility function isU(x,y,z)=x+z^1/2f(y), wherezis the number of tapes she buys,yis the number of tape recorders she has, andxis the amount of money she has left to spend.f(y)=0 ify<1 andf(y)=24 ifyis 1 or greater. The price of tapes is $4 and she can easily afford to buy dozens of tapes. How many tapes will she buy?

a.8

b.9

c.20

d.16