Aha! I think I have it.
Since T is a linear transformation, T(0)=0, and T(au+ bv+ cw)=aT(u)+bT(v)+cT(w)=0
Since there exists scalars, a, b, c, not all 0, such that the above equation is true, S` is linearly dependent.
I can't believe I missed that even after ZioX's advice, guess I...