Answer:
(a) - Thinking about a story as you read it
I am pretty sure it is "became". Hope that helped :)
Human activities cause increased nitrogen deposition in a variety of ways, including
<span>burning of both fossil fuels and forests, which releases nitrogen into the atmosphere
fertilizing crops with nitrogen-based fertilizers, which then enter the soil and water
ranching, during which livestock waste releases ammonia into the soil and water
<span>allowing sewage and septic tanks to leach into streams, rivers, and groundwater</span></span>
