單項選擇題燃?xì)夤艿来┰借F路時應(yīng)加套管,套管內(nèi)徑應(yīng)比燃?xì)夤艿劳鈴酱螅ǎ┮陨稀?/strong>

A.50mm
B.70mm
C.100mm
D.150mm