In Exercises 21 and 22, mark each statement True or False. Justify each answer.
a. A linearly independent set in a subspace $H$ is a basis for $H .$
b. If a finite set $S$ of nonzero vectors spans a vector space $V,$ then some subset of $S$ is a basis for $V$ .
c. A basis is a linearly independent set that is as large as possible.
d. The standard method for producing a spanning set for Nul $A,$ described in Section $4.2,$ sometimes fails to produce a basis for Nul $A$ .
e. If $B$ is an echelon form of a matrix $A$ , then the pivot columns of $B$ form a basis for $\operatorname{Col} A .$