I will throw a birthday party to observe my 18th birthday.句中observe一词的意思是（）。A、观察B、听课C、遵守D、庆祝生日

I recently went to a charity party. At the end of the visit our host told us that the following Monday was his birthday. He asked21, as a gift to him, we would do something kind for someone else on that day. I thought that was a terrible birthday 22 !
The following Monday, I saw my neighbor, a new mother, in the garden with her baby, I went outside to say"hello" to her. During the talk, she told me, not in a complaining 23 but just as a matter of fact, about the sleeplessness and the challenges to get anything done with a baby followed by. I 24 the charity party host's request and said,"Hey!Why don't 25 watch your baby for an hour!I will just hang out with him here in the back garden 26 you go in and take an hour to yourself."
She was so surprised that she almost cried."Are you 27 Would you be able to do that ""Of course! " I said. "I'd be happy to! " An hour later she came outside with a smile on her face."I have 28 so much done! " she told me, and I told her that I had sung every kid's song I knew and had a good time hanging out with the baby, too. And I was so happy to see her smiling like that.
It was one of the best 29 I've ever given, and it has given me the desire to ask the same 30 all my friends this year. I know it will make me feel great to know my friends are out there sharing their wisdom and time with people who can really use it.
I recently went to a charity party. At the end of the visit our host told us that the following Monday was his birthday. He asked21, as a gift to him, we would do something kind for someone else on that day. I thought that was a terrible birthday 22 !
The following Monday, I saw my neighbor, a new mother, in the garden with her baby, I went outside to say"hello" to her. During the talk, she told me, not in a complaining 23 but just as a matter of fact, about the sleeplessness and the challenges to get anything done with a baby followed by. I 24 the charity party host's request and said,"Hey!Why don't 25 watch your baby for an hour!I will just hang out with him here in the back garden 26 you go in and take an hour to yourself."
She was so surprised that she almost cried."Are you 27 Would you be able to do that ""Of course! " I said. "I'd be happy to! " An hour later she came outside with a smile on her face."I have 28 so much done! " she told me, and I told her that I had sung every kid's song I knew and had a good time hanging out with the baby, too. And I was so happy to see her smiling like that.
It was one of the best 29 I've ever given, and it has given me the desire to ask the same 30 all my friends this year. I know it will make me feel great to know my friends are out there sharing their wisdom and time with people who can really use it.

A.plan
B.idea
C.cake
D.party

I don't quite follow what she is saying.

A:observe
B:understand
C:explain
D:describe