阅读下面短文，掌握其大意，然后从各题所给的四个选项（A、B、C和D）中，选出最佳选项。
Every fast-food employee knows that the drive-through window is the worst position. The window sticks with constant ice; the roar of engines hurts your 1; your words are lost in the howling wind.
On a 2 afternoon, everything changed.
Every once in a while, the sub-zero temperatures seal a 3 windows shut. Drivers don't exactly enjoy having to stand outside in the cold, 4 their orders into a speaker box, when they expect to drive through on their heated leather seats. In such cases, most customers tend to show their annoyance to the employees.
This woman was 5.
“I'll get the next car's 6 as well,” she said as she came up to the window to pay. She stood outside, 7 much snow on her hair. Though she was obviously freezing, her bright 8 lit up her face like a fire.
“You can't 9 their drinks,” I said, confused and tired.
“No, but I'll buy them,” she said. “Pay it forward and all that.”
Completely puzzled, I charged her as 10, and when the next customer arrived at the window I explained what had just happened. I watched as his 11 changed — first angry to be out in the cold, then 12 at the random act of kindness, and finally, delighted by his 13.
“I suppose I'll pay for the next order then,” he replied, nodding and waving at the impatient driver 14 him. He 15 over the cash and received his pre-paid hot drink. The trend continued. Customers arrived annoyed, only to leave 16 and pleased. Some were shocked to spend much more than they had expected, 17 others ended up receiving their order for less than half the price.
Five vehicles passed, then ten, then twenty. No one refused to pay. Customers stood at my window 18 a fist-full of change to buy drinks for a complete stranger. Cars drove off, honking (鸣笛) and 19 their thanks.
It only takes one customer, one person, to change the entire 20 of traffic. It only takes one moment, one smile, to warm up even the coldest of days.
2MoO3 + 4SO2； ②MoO3 + 2N
H3·H2O = (NH4)2MoO4 + H2O；
MoO3 + H2O；