Write a multi-choice question for the following article, with the given choices and answer: Article: John: Here's a good shop. Shall we buy mother's birthday present here? Mary: Yes, that's a good idea. Shall we go inside? Tom: No. Let's look in the window. Shall we buy her a sweater? Anne: Er, no. It'll soon be summer. Let's buy her a blouse to wear. There's a nice one in the window. John: No, she has two blouses. Let's buy a ring. Mary: Oh, no! They're diamond rings. Look at the price. The cheapest is $15. John: A real diamond ring is at least $500.They only look like diamonds. Tom: Shall we buy a table? It's only $15. Anne: It doesn't look good, just like a big box. Mum likes chairs. Tom: But they haven't any here. Mary: What about a pen? So cheap! Only $10. John: She has a lot of pens and pencils. All of them are new. Tom: Oh, look here. These flowers are beautiful. Mary: They aren't real and will never die. John: And they're the cheapest of all these things. Yes, let's buy them. Anne: All right. Options: A Because it looks so bad. B Because Mother has many pencils. C Because it's too cheap. D Because Mother doesn't need pens. Answer: D Because Mother doesn't need pens. Question:
Why don't they buy the pen?