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 fix or replace your preamp tubes and power tubes [title] Identify the problem. [step] The screws will be located on the side of the head, or on the back of the cage itself. [title] See if your preamp tubes are the problem. <sep> (A) [step] Some tubes have screw connections, and while others have different screws on them, some only have one screw on them. You will need to be able to remove these screws using a screwdriver, and getting the screws unscrewed will give you an idea of what you need to remove. (B) [step] If they are, remove them to find out exactly what's causing them to malfunction. [substeps] If they are, leave them alone. (C) [step] Firmly grasp the tube with one hand and press down on the clamp to provide room for the tube to move. [substeps] Gently rock the tube back and forth while lightly pulling upwards. (D) [substeps] Look around in the cage and flex your tubes, as an indication of disfunctionality, and try to find the weakest tubes. Some tubes will have 1 hole, while others will have two.
C