Graphite fibers are composed of repeating units of graphitic layers.

Identify the incorrect statements from the following set.$(2)$

$($a$)$ Chains within the basal plane are held together by metallic bonds

$($b$)$ Adjacent layers are held together by covalent bonds

$($c$)$ Graphite fibers are not as strong in the direction perpendicular to the fiber axis as they are in the direction parallel to the fiber axis

$($d$)$ Chains within the basal plane are held together by ionic bonds