$11.$ Which method is commonly used to improve the fuel economy of internal combustion engines?

$-$ A$)$ Increasing engine displacement

$-$ B$)$ Implementing start$-$stop systems

$-$ C$)$ Reducing compression ratio

$-$ D$)$ Using higher$-$octane fuel

$12.$ What is the primary source of carbon monoxide $($CO$)$ emissions from internal combustion engines?

$-$ A$)$ Incomplete combustion of fuel

$-$ B$)$ Evaporation of fuel

$-$ C$)$ High$-$temperature combustion

$-$ D$)$ Lubricating oil burning