单项选择题
The exhaust ports are located around the lower part of the cylinder liner, （）the products of combustion are driven by scavenging air.
A.in which B.by which C.through which D.from which
单项选择题 The main engine is cooled with fresh water （）is added suitable small quantities of chemicals to prevent corrosion.
单项选择题 （）during the last repair?
单项选择题 Hardly had the engine started （） the temperature of the cooling water suddenly rose up to 40℃.
