The Bhopal gas tragedy in $1984$ was caused by the leakage of ......... .

How many sentences are correct?
$(1)$ $CNG$ burns most efficiently.
$(2)$ $CNG$ cannot be adulterated.
$(3)$ $CNG$ is cheaper than petrol and diesel.
$(4)$ In the $1990$s,Delhi ranked $4$th among the $41$ most polluted cities of the world.

Lead $(Pb)$ causes:

$LPG$ cooking gas is

What is the ascending order of $BOD$ for the following: Sewage $(S)$,Paper mill effluent $(PE)$,Distillery effluent $(DE)$,and Sugar factory effluent $(SE)$?

Identify the incorrect statement.