阅读理解
At a talk to some businessmen earlier today, one fellow asked me, “Would you give money to a homeless person, even when you know he's going to use it to buy alcohol?” I replied, “If all was giving was money, it'll be one thing. But for me, whenever I do give money, that's just the wrapping (包装纸) . The real gift is hidden inside – it's love. And l haven't ever found any reason to limit gifts of love.”
By the end of the night, I decided to get some pizza. In front of me in line was a homeless-looking man. He was counting the few dollar bills in his hand, over and over again, until it was his turn to order. Just then, he told the young woman behind the counter, “I'll have the full cheese pizza.” “Full?” she countered. “It's really big.” she added, although what she really meant to say is that it'll be too expensive for him. “Yeah, the full.” “$18.65,” she submitted. The bills he was counting weren't going to make it. I wasn't intending to get the whole pizza for him. But thinking back to my conversation with the business man, l realized that money was really just the wrapping.
I made my move. “Can you charge that pizza with my order?” I said to the cashier. “Really?” the homeless-looking man said. “Really?” the young woman behind the counter repeated. “Yes, really.” That disheveled (凌乱的) man and I had a sweet silent moment. To his silent question of why I was doing it, I added, “Just pay it forward for someone else.” And he said, “Well, you know what? I was actually treating these five homeless guys out there.” He was actually paying it forward, in the first place. What an unexpected fortune to get a chance to be a part of a man's act of kindness.
根据短文内容，选择最佳答案，并将选定答案的字母标号填在题前括号内。
3Z(g) ，此反应达到平衡的标志是