Design a flexible pavement for the ESALs $=2,700,000,M_R$ of soil $=7,800,$ reliability $=0\mathrm{.}95,$ standard deviation $=0\mathrm{.}45,$ and PSI drop $=2.$ What's the approximate structure number from the nomograph?

A$.3\mathrm{.}7$

B$.4\mathrm{.}5$

C$.5\mathrm{.}0$

D$.5\mathrm{.}5$