单选题 When is the main subject discussed in the text? [A] Dettling designed the first green building in the U. S. [B] Energy shortage calls for buildings of new design. [C] Green building help save environment.

【正确答案】 C

【答案解析】主旨题。文章主要讲了绿色建筑的种种优点及未来，可知它主要是能保护环境，节约能源。所以[C]正确。