In each case determine whether U is a subspace of R3.

a. U ={(1, s, t) | s and t in R}.
b. U = {(0, s, t) | s and t in R}.
c. U={(r,s,t) |r,s, and t inR, − r + 3s + 2t = 0}.
d. U={(r,3s,r−2) |r and sinR}.
e. U={(r,0,s) |r²+s²=0,r and s in R}.
f. U={(2r,−s²,t) |r,s, and t in R}.