4. How many pairs of letters remain unchanged when encryption is performed using the following digraphic ciphers
a)
$$\begin{array}{l} C_{1} \equiv 4 P_{1}+5 P_{2}(\bmod 26) \\ C_{2} \equiv 3 P_{1}+P_{2}(\bmod 26) \end{array}$$
b)
$$\begin{array}{l} C_{1} \equiv 7 P_{1}+17 P_{2}(\bmod 26) \\ C_{2} \equiv P_{1}+6 P_{2} \quad(\bmod 26) \end{array}$$
c)
$$\begin{array}{l} C_{1} \equiv 3 P_{1}+5 P_{2}(\bmod 26) \\ C_{2} \equiv 6 P_{1}+3 P_{2}(\bmod 26) ? \end{array}$$