问题如下：
The privatization of an existing hospital is best described as:
选项：
A. a greenfield investment.
B. a brownfield investment.
C. an economic infrastructure investment.
解释：
B is correct.
Investing in an existing infrastructure asset with the intent to privatize, lease, or sell and lease back the asset is referred to as a brownfield investment. An economic infrastructure asset supports economic activity and includes such assets as transportation and utility assets. Hospitals are social infrastructure assets, which are focused on human activities.
基建投资有两种分类方式：
一种按照被投资物的状态：即未建/在建(greenfield)或者已经建成(brownfield),
另外一种按照被投资物的用途：
economic or social use, economic就是来支持经济活动的，例如铁路，公路
而social use就是社会活动的，例如医院、学校等。
题中的hospital对应social use，同时又是existing, 所以是brownfield.
C为什么不对呢，的确是社会型投资
