$5.$ Which emission control method involves injecting a substance into the exhaust stream to neutralize pollutants?

$-$ A$)$ Catalytic conversion

$-$ B$)$ Exhaust gas recirculation $($EGR$)$

$-$ C$)$ Diesel particulate filter $($DPF$)$

$-$ D$)$ Selective catalytic reduction $($SCR$)$

$6.$ What is the primary purpose of exhaust gas recirculation $($EGR$)$ in internal combustion engines?

$-$ A$)$ To increase engine power

$-$ B$)$ To improve fuel efficiency

$-$ C$)$ To reduce NO$$ emissions

$-$ D$)$ To enhance engine cooling