Given the task definition and input, reply with output. In this task, you are given a context and four options. Each option is a suggested ending for the context. You should read the context and pick the best ending for the context. Please answer with "A", "B", "C", and "D".

[header] How to prevent fertilizer runoff [title] Use phosphorus-free fertilizer. [step] Most fertilizer bags will have a ratio of nitrates-phosphorus-potassium printed on the outside of the bag. Look for a number like 32-0-25, where the middle number, denoting phosphorus content, is zero. <sep> (A) A typical fertilizer bag contains 9 nitrates per square foot. [substeps] Phosphate bars can be purchased at gardening and home improvement stores. (B) The different numbers on the bag are also graded according to time of year. [substeps] Phosphate refers to the ratio of nitrogen and nitrogen by the caratomic (or cubic diatomic) process. (C) [substeps] Phosphorous is the largest source of concern when it comes to controlling fertilizer runoff, because it's dangerous to aquatic plants and wildlife. [title] Clean up grass, leaves, and other yard debris. (D) In general, you should use nitrate-free fertilizers. Nitrate levels will fluctuate over time, though nitrates can come in regular amounts, so you should use a balance meter to calculate nitrate levels for all fertilizers.
C