单项选择题 The main bearings are provided with thin-walled （）, lined with a copper-lead alloy.

单项选择题 The main bearing shells often have a thin （） of lead or indium to provide a layer giving protection against corrosion.

单项选择题 The main bearing shells are made of steel with a lining of bearing metal which may be white metal, copper-lead or aluminum-tin alloy What is the meaning of “which” in the sentence above?（）