---What are you mailing, Ryan?---A textbook ______ a new method of teaching physics．I want my friends to take a look at it．A.is based onB.based onC.basing uponD.which based upon
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B解析从语境推出答语是一省略句，回答的是mail的宾语what，所以A textbook之后是限定修饰部分，排除A项；而base是及物动词，跟textbook构成动宾关系，排除D项，也不能用现在分词作定语，C亦不对。