that作关系代词的用法特点：
1） than可作关系代词引导定语从句，在从句中一般作主语。
2） than前面的主句需有形容词的比较级形式。
a.Don't drink more wine than is good for health.
不要饮酒过量，适量饮酒有益于健康。
b.Don't give him more money than is needed , since money will burn a hole in his pocket.
不要多给他钱，他需要多少就给多少，因为他有钱就会花掉。
c.Today we know a great deal more about the sea than was known in ancient times.
d.There ought to be less anxiety over the perceived risk of getting cancer than_________in the public today.
A. exists
B. exist
C. existing
D. existed
正确答案：A
分析：这里的than指anxiety，且根据句末today可判定从句的时态为一般现在时，所以选A。
译文 ：人们不应该像如今这样担心患上癌症。
3） than也可在从句中作宾语。
For now, however, it does appear that the economy can sustain a higher growth rate than most people thought impossible just a year or two ago. In that limited respect, at least, we appear to be in a "New Economy”.
不过，目前我们的经济确实实现了持续的、较快的增长，在一两年前绝大多数人认为该增长率是不可能实现的。所以，仅此而言，我们正处于一个“新经济时代”。