正确答案：
As the price of glass came down, greenhouses became ever more elaborate and grand, culminating in Joseph Paxton’s Crystal Palace, a 990,000-square-foot conservatory built London’s Great Exhibition of 1851.
解析：
题目询问建造大型温室的实际原因，分析文章可知材料最后一句话是对这一原因进行的叙述。
