Which two numbers should be interchanged to make the given equation correct?
$$81 + 31 \div 9 - 7 = 33$$

$$81\ +\ 31\ \frac{\ 9}{ }\ -\ 7=33$$

9 and 7

$$81\ +\ 31\ \frac{\ 7}{ }\ -\ 9=33$$

$$112\ \frac{\ 7}{ }-9=33$$

$$16\ -\ 9=33$$

$$7=33$$

31 and 9

$$81\ +\ 9\ \frac{\ 31}{ }-7=33$$

$$90\ \frac{\ 31}{ }-7=33$$

$$90\ \frac{\ 24}{ }=33$$

$$3.75=33$$

31 and 81

$$31\ +\ 81\ \frac{\ 9}{ }-7=33$$

$$31\ +\ 9\ -\ 7=33$$

$$40-7=33$$

$$33=33$$

81 and 7

$$7\ +\ 31\ \frac{\ 9}{ }-81=33$$

$$38\ \frac{\ 9}{ }-81=33$$

$$4.22-81=33$$

$$-76.78=33$$

Hence Option C is the correct answer L.H.S=R.H.S