Why is recycling paper cheaper then recycling plastics, metals and glass?

In a normal, well-run landfill, paper bags do not biodegrade any faster over at least 40 years than plastic.” ... “They're three to five times bulkier than plastic, and you can see that yourself at the grocery. Landfills are closing down because they're full. From that perspective, plastic is much better than paper.”

b is the best answer for this question.