$3.(15$ marks each$)$ Assume the $\backslash ((7,3)\backslash )$ single$-$walled carbon nanotubes $($SWCNTs$)$ in a nanotube bundle prepared by chemical vapor deposition are well aligned. Answer the following questions:

$($a$)$ Are these chiral or zigzag or armchair SWCNTs$?$

$($b$)$ What is the diameter of the SWCNTs$?$ The C$-$C bond length is $\backslash (\backslash$mathrm$\{$a$\}=0\mathrm{.}143\backslash$mathrm$\{$~nm$\}\backslash ).$

$($c$)$ Explain briefly why the mechanical strength of $\backslash ((7,3)\backslash )$ SWCNTs estimated from the tensile curves for the nanotube bundle is much different with that directly measured on an individual $\backslash ((7,3)\backslash )$ SWCNT$.$