Definition: 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".
Input: [header] How to determine which pool algaecide and pool clarifier is best for your swimming pool [title] Add preventative algaecide to your swimming pool on a weekly basis if you regularly experience problems with algae growth. [step] If you have never had an algae problem in your swimming pool, there is no need for the algaecide. [title] Realize that if you have algae right now, you may need to use an algaecide. <sep> (A) [title] Know how to know when to seek professional assistance. [step] Algae growth happens not commonly in pools. (B) [step] Algae tends to get in the way of chlorinating materials (if water has built up in your pool). Algae often gets stuck in the tips of pool grates. (C) [step] Some algaecides are preventative and others are used to treat an algae problem. Once algae has grown in your swimming pool, only a treatment algaecide combined with shocking will kill the algae. (D) [step] The algaecide and pool clarifier often contain saline. These saline tablets are, however, available to purchase at pool shops or online.
Output:
C