问下英语达人 下面三个句子如何区分than 后面是否用be 动词

问下英语达人 下面三个句子如何区分than 后面是否用be 动词
1.According to a survey of graduating medical students conducted by the association of American Medical Colleges,minority graduates are nearly four times more likely than are other graduates in planing to practice in socioeconomically deprived areas
2.Plants are more efficient at acquiring carbon than are fungi,in the form of carborn dioxide,and converting it to energy-rich sugars
3.According to public health officials,in 1998 massachusetts became the first state in which more babies were born to women over the age of thorty than under it
什么时候THAN 后面用Be 动词,什么时候主谓都省略~
不好意思,第2句打错了~是plants are more efficient at acquiring carbon than fungi~