Q$4(10$ points$)$: Consider the seven structures shown below. Determine if each structure is unstable, externally statically determinate, or externally statically indeterminate. If the structure is externally statically indeterminate, state the degree of indeterminacy. If the structure is unstable, describe in words and with a drawing, the mechanism of instability.

$($a$)$

$($b$)$

$($d$)$