GWD 42-19

According to some experts, carbon dioxide is accumulating in the atmosphere so rapidly, largely because of the burning of fossil fuels, that should the present rate of buildup continue, the global concentration of this poison will double by the end of the next century.

(A) that should the present rate of buildup continue, the global concentration of this poison

(B) should the present rate of buildup continue, the global concentration of this poison

Why is the answer A?

Thank you!

明显这里的that是指代 前面的carbon dioxide is accumulating in the atmosphere，难道should的主语是句子时，要用that代？同问

看清句子的结构：carbon dioxide is accumulating in the atmosphere so rapidly, largely because of the burning of fossil fuels, that

so rapidly... that