1) Incorrect, because a + b + c + d can be zero in many ways other than a, b, c and d musteach be a null vector e.g., if the vectors are in different directions, then their resultant willbe zero. 2) Correct, since a + b + c + d = 0, a + c = -(b + d) 3) Correct, since a + b + c + d = 0, a = -(b + c + d). 4) Correct, since a + b + c + d = 0, a + (b + c) + d = 0