A flexible pavement system is to be designed using the AASHTO structural number design method

with the following criteria:

If the minimum thicknesses of the aggregate base and subbase are used, the required thickness of

the asphalt surfacing $($in$.)$ is most nearly

a$.2\mathrm{.}0$

b$.2\mathrm{.}5$

c$.3\mathrm{.}0$

d$.3\mathrm{.}5$