Their new bridge is being built now, which is said to be ______the old one.[ ]

A. the length three times of              

B. three times the length of

C. three times as longer as                

D. three times the length as

天然气与空气混合遇火发生爆炸时的天然气的浓度范围叫做爆炸极限，而天然气与空气混合遇火发生燃烧时天然气的浓度范围不叫做爆炸极限。