阅读下面短文，从短文后各题所给四个选项（A、B、C和D）中，选出可以填入空白处的最佳选项，并在答题卡上将该项涂黑。
Last Christmas while staying with my parents, I 1 across some old love letters that my parents wrote to each other. These letters were all piled up in a basket, dirty and 2 with dust.3 to read and sort them, I asked them if I could take the letters back to my Illinois home. They agreed.
As I carefully opened each letter, all of them delicate with age, I discovered a new page 4unknown to me in this private chapter of my parents' lives.
My father used to 5 in the army. So his letters were full of frontline 6 of the things about the war. Each of my mother's letters was marked 7 her 1944 dark red lipstick kiss. I was 8 to these letters like a magnet (磁铁).
Just six weeks after our Christmas visit, Daddy became very 9and was hospitalized. This time, he was fighting a 10 kind of war. As I sat by his bedside, we discussed the 11 . He told me how much receiving those lipstick-kissed letters had 12to him when he had been so far from home.
It so happened that the next day would be February 14. From the 13letters I chose the card my father had sent Mother in 1944 and brought it to my father's bedside.
At his bedside, I joked with him, saying 14, “Today is Valentine's Day, don't you want to send Mother a present?” He became more 15when I handed him the old 16. He carefully opened it and took out the card, and when he 17 it, his eyes were filled with tears.
My father, in a voice tight with18read the loving19he'd sent to my mother fifty-six years earlier. And this time, he could read it to her 20.
2X+C(金刚石)。则X的化学式为（）
