$235.$In a packed tower, the value of HETP equals HTUOG, when the equilibrium and the operating lines are $($where$,$ HETP $=$ height equivalent to a theoretical plate HTUOG $=$ overall gas phase height of a transfer unit$).$

Straight

Parallel

Both $($A$)$ & $($B$)$

Neither $($A$)$ nor $($B$)$