Please help with the following homework question regarding negative externalities:

Greenhouse gas emissions are considered as negative externalities due to the potential impact on climate change. There are various types of policies proposed to cut greenhouse gas emissions such as cap-and-trade and carbon tax. Why do these policies place the requirements of dealing with greenhouse gas emissions on firms such as power plants and cement manufacturers, rather than individual citizens? Hint: you need to talk about the cost of negotiation and cost of making the adjustment