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".

Example input: [header] How to create a christmas tree forest [title] Consider the type of tree you want to plant. [step] There are a wide variety of christmas trees and include fir, pine, spruce, cypress, and cedar. These trees thrive in different climates and look different from one another. <sep> (A) It is important to research the tree before setting up your tree to try it out to make sure you can write the tree's growing season so that you can get the most supplies you need. If you're just interested in having a tree stand out, then make sure to check for the following : [substeps] Spruce trees. (B) [substeps] Trees green may be used in a mix of green and evergreen. Greens, such as pintos and olive oil, may be more delicate than these. (C) Where the trees grow together is up to you. [substeps] Spruce trees light up the night for hours in the morning and drown out the morning sunrise between afternoons. (D) Think about where your tree forest will be located and choose a tree that can thrive in that climate. [substeps] Balsam firs are a popular christmas tree species that grow in the northern united states and canada, in hardiness zones 3-5 or states like minnesota, north dakota, and montana.
Example output: D
Example explanation: This is a good example. Only option D is about Christmas trees. So, it's the correct option.
Q: [header] How to identify atrial fibrillation [title] Monitor any breathlessness you experience. [step] When your heart starts beating quickly, as it does when you have atrial fibrillation, the rest of your body gets less blood supply because your heart is beating less efficiently. When your body's blood supply slows down, oxygen levels in your blood decrease, which will make you feel like you can't catch your breath. <sep> (A) [substeps] Dilation is when the blood vessels in your heart contract. Stopping them helps blood flow more effectively and allows more blood to be drawn from the heart. (B) [substeps] Your heart isn't pumping as quickly as normal when you have atrial fibrillation. This can make it harder for the heart to work smoothly. (C) [substeps] You may experience this feeling of breathlessness when you are climbing the stairs, exercising, or simply just walking. [title] Notice if you feel like your heart is pounding. (D) But even if your heart stop beating rapidly, that still means your heart is not working efficiently or is working too fast. This is called atrial fibrillation.
A:
C