问题
单项选择题
______ a substance can be synthesized.
A．Upon identifying the basic components of it
B．Once chemically analyzed
C．When gas chromatography is used
D．If spectroscopy is adopted
答案
参考答案：B
解析：[分析] 细节题型。 见第二段倒数第二句(见75题答案)，所以选项B为答案。