(1 point) which of the following statements are true? a. the equation ax=b is referred to as a vector equation. b. if the columns of an m×n matrix a span rm, then the equation ax=b is consistent for each b in rm. c. if the equation ax=b is inconsistent, then b is not in the set spanned by the columns of a. d. if a is an m×n matrix and if the equation ax=b is inconsistent for some b in rm, then a cannot have a pivot position in every row. e. any linear combination of vectors can always be written in the form ax for a suitable matrix a and vector x. f. if the augmented matrix [ a b ] has a pivot position in every row, then the equation ax=b is inconsistent.