thE grEEnhousE EFFECt rEsultED hunDrEDs oF million yEArs Ago whEn _________.
A．thE AtmosphErE ChAngED DrAmAtiCAlly
B．oxygEn lEvEls on thE EArth BECAmE lowEr
C．thE sEA lEvEls BECAmE lowEr grEAtly
D．thE sEDimEnt gAvE oFF Enormous CArBon AnD mEthAnE