$4.$ A stock currently sells for $100$ and can rise or fall by $10$ per cent for each of the hext two periods. The risk free rate of interest is $$ per period. The value of a call option with strike price of $98$ is

a$)6\mathrm{.}25$

b$)5\mathrm{.}50$

c$)2\mathrm{.}00$

d$)7\mathrm{.}50$