--- _______ do you have piano lessons?

--- Once a week.

A．How many

B．How long

C．How often

D．How soon

双管同沟铺设沟底安装时，管沟沟底宽度计算公式为：a=D₁+D₂+S+C，设第一根管道外径D₁=0.1m，第二根管道外径D₂=0.2m，两管之间的设计净距S=0.7m，工作宽度C=0.6m，则沟底宽度为（）。

A．1.0m

B．1.3m

C．1.6m

D．2.0m