If A = $\begin{bmatrix$}[r] a & b \end{bmatrix} \text{ and } \begin{bmatrix}[r] c \ d \end{bmatrix}, then :

1. only matrix AB is possible

2. only matrix BA is possible

3. both matrices AB and BA are possible

4. both matrices AB and BA are possible, AB = BA

Matrix A = $\begin{bmatrix$}[r] 6 & 9 \ -4 & k \end{bmatrix} \text{ such that } A^2 = \begin{bmatrix}[r] 0 & 0 \ 0 & 0 \end{bmatrix}. Then k is :

1. 6

2. -6

3. 36

4. ±6

Which of the following is not a geometric progression ?

1. $\dfrac{1}{3}, 1, 3, 9$

2. $\dfrac{1}{5}, \dfrac{1}{5}, \dfrac{1}{5}, \dfrac{1}{5}$

3. -2, 4, -8, 16

4. 2, 0, 4, 0, 8, 0

In the adjoining diagram, G is the centroid of △ ABC. A(3, -3), B(2, -6), C(x, y) and G(5, -5). The coordinates of point D are :

1. (2, -6)

2. (3, -6)

3. (6, -6)

4. (10, -6)