$4.$ If investors require an extra $0\mathrm{.}5$ per cent each year for investing in two year bonds instead of $1$ year bonds, if the one year spot rate is $1$ per cent and the two year spot rate is $1$ per cent, the expected spot rate next year is

a$)1\mathrm{.}0$

b$)1\mathrm{.}5$

c$)0\mathrm{.}5$

d$)0\mathrm{.}0$