I saw this exercise and the question asks to add or omit an 's' where appropriate.
"To sum up, as we have seen it is quite clear that using computer(-s/ __) is a double-edged sword."
One of my friends said that I should add an 's' after computers because we are talking about computers in general. Another friend said that we add an 's' because of the word 'using' is used. Which reason is correct? Why?