第三阶段是20世纪上半期。

The third stage is half of the 20th century.

20世纪上半期，血吸虫病泛滥于长江流域。

During the first half of the 20th century, schistosomiasis was widespread in the Yangtze River Valley.

转换资产的时机大概在2010年的上半期。

The time for switching will probably be in the first half of 2010.