A $W1850$ section with four lines of bolts, two lines per flange and threes bolts per line was subjected to pure tensile force. The efficiency factor for this section is:

a$)U=1\mathrm{.}00$

b$)U=0\mathrm{.}90$

c$)U=0\mathrm{.}85$

$($d$)U=0\mathrm{.}75$