Fathers often feel __________ their duty to earn more money for their family.

A．that

B．it

C．one

D．what

根据《个体防护装备选用规范》（GB/T11651-2008）中对作业类别的规定：密闭场所作业是指在空气不流通的场所中作业，包括在缺氧即空气中含氧浓度小于（）％和毒气、有毒气溶胶超过标准并不能排除等场所中作业。

A.20

B.19

C.18

D.17