Which of the following is incorrect?

Option $\left(C\right)$ & $\left(D\right)$ are theorems

Hence, $\left(C\right)$ & $\left(D\right)$ are correct
For option $\left(A\right)$,

\left(p ⇔ q\right)$≡ p ⇔$\left( q\right)$<br />$≡ p ⇔ q$<br />Hence, \left(A\right)$ is correct
For option \left(B\right)$,</p><p>When$p$is true and$q$is false then$p ⇔ q$is true and$p ⇒ q$is false</p><p>Hence, option \left( B\right)$ is incorrect