单选题
（ ）,photo synthesis is the ultimate source of food for almost all organisms on earth.
AIt is an extremely important process
BAn extremely important process,it is
CThat an extremely important as
DAn extremely important process
正确答案
答案解析
该题考查句子结构。题干中photo synthesis意为“光合作用”，根据选项可知，空格处所填内容是photo synthesis的同位语形式，选项A、B不合句子结构，C选项不合说法。故答案为D。