Check whether the following processes X, are martingales w.r.t. {F}: (i) X₁ = B₁+4t (ii) X₁ = B (iii) X₁ = t² B₁ - 2 sBds (iv) X₁ = B₁(t)B2(t), where (B₁(t), B2(t)) is 2-dimensional Brownian motion.