Which of the Following Statements Best Describes a CPR 36

Which of the following statements best describes a CPR 36 offer?

A) Either a written or verbal offer designed to show a willingness to settle and which can be shown to the court at any time.
B) A written 'without prejudice save as to costs' offer that is expressed to be open indefinitely but if accepted within a relevant period of at least 21 days, there is an implicit order for costs in favour of the receiving party.
C) A written offer to compel settlement made after the issue of proceedings.