You will be given a definition of a task first, then some input of the task.
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 use a sleep number bed [title] Use a platform or board frame for the bed. [step] The base of the bed must be firm and solid so the mattress is well supported. Avoid using a traditional box spring or a slatted bed frame as they will not be supportive enough. <sep> (A) Instead, make sure the front or back of the bed frame is supported. [title] As for the slide on top, top only supports the bed. (B) [substeps] Use pillows and cushions if you can to bring a feel to the style of the bed. [title] Select a platform or board frame to place the bed on. (C) Make sure the platform or board frame is big enough to fit the mattress and is at the ideal height for you. [substeps] If you do not want to have to worry about finding the right frame for the mattress, you can buy a base for the bed when you buy the mattress at an extra charge. (D) [title] Purchase a bed rail for the base. [step] The platform at the base should be the height that the foot of the mattress is at.
Output:
C