_______,but he still could not understand it.

A．He had been told many times

B．Having been told many times

C．Told many times

D．Although he had been told many times

大气中的氧和二氧化碳含量能保持相对稳定，正是[ ]

A．光合作用的结果

B．呼吸作用的结果

C．大气中氧含量多的结果

D．氧大量消耗的结果