Determine which of the following sets of vectors x = (x₁, x₂, . . . , x_n)^T are subspaces of Rⁿ:

(a) All equal entries x₁ = · · · = x_n; 

(b) All positive entries: x_i ≥ 0; 

(c) First and last entries equal to zero: x₁ = x_n = 0; 

(d) Entries add up to zero: x₁ + · · · + x_n = 0; 

(e) first and last entries differ by one: x₁ − x_n = 1.