Another basis and dimension question

Consider the set where

a) Find a basis B for span

For this question, would you just write the columns out in a matrix, then reduce into reduced row-echelon form.

The leading (pivot) columns would then form a basis B

b) Explain why belongs to span span . Write as a linear combination of B.

Since the vectors in part (a) form a basis, does that mean that any vector in can be written as a linear combination of the vectors in B

c) Suppose the matrix A has columns . what is the dimension of the column space of A?

I don't really understand what column space is?