On Sundays there were a lot of children playing in the park, ________ parents seated together joking．

A．their

B．whose

C．which

D．that

目前地球大气的CO₂浓度增高，由于温室效应，地球表面的温度也有所提高，这将导致土壤沙漠化等恶果，导致CO₂增高的主要原因是[     ]

A．自然因素破坏了生态平衡

B．工业污染和自然资源的不合理利用

C．大量生物物种灭绝

D．人口剧增，呼出CO₂增多