Question 18
In a $4\times4$ table, there is a symbol in each square: a $\circ$, a $\bullet$, a $\triangle$ or a $\triangledown$. There is exactly one symbol of each kind in each row, each column and each of the two long diagonals. The symbols in four of the squares are shown in the diagram. What symbol should be placed in the square marked with a question mark?
