I have the following question to you  

Question: In my recent document you edited, you wrote the sentence as follows.

"Characterizing and investigating the ways in which NPs behave in environmental mediums is necessary for determining the overall environmental impact of NPs." 

Here, I think that since the subject "Characterizing and investigating" is plural, the verb "is" should be "are", but I don't understand why you wrote the verb as "is"

please clarify this soon. Let me know