[ndnSIM] How to know the content category of the packet

[L]

Hello, 
    I use ndn-consumer-zipf-mandelbrot.cpp and set the number of content to 3. My understanding is that every interest package sent by the consumer belongs to one of the three contents, such as A, B or C. So, when a consumer sends an interest package, how can I know that the interest package belongs to A, B or C? Similarly, the returned data packet should also be divided into three, how do I know which one the data packet belongs to?
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