$1)$ How many elastic constants do the most anisotropic elastic solids have? $(5$ scores$)$

A$.1$

B$.2$

C$.15$

D$.21$

$2)$ Has elasticity theory found its applications in which of the following engineering areas? $(5$ scores$)$

A$.$ Aeronautical and aerospace engineering

B$.$ Civil engineering

C$.$ Mechanical engineering

D$.$ Materials science and engineering

$3)$ In spherical coordinates, unit vectors are functions of which of the coordinate variables? $(5$ scores$)$

A$.$ Spherical latitudinal coordinate $($$)$

B$.$ Spherical longitudinal coordinate $($$)$

C$.$ Spherical radial $($R$)$

D$.$ All

$4)$ Which of the following are isotropic tensors, so that their components do not change under

coordinate transformations? $(5$ scores$)$

A$.$ Levi$-$Civita symbol

B$.$ Any vector

C$.$ Kronecker delta

D$.$ Any scalar