根据下面资料，回答
It is well that young men should begin at the beginning and occupy the most subordinate positions. Many of the leading businessmen of Pittsburgh had a serious responsibility thrust upon them at the very threshold of their career. They were introduced to the broom, and spent the first hours of their business lives sweeping out the office. I notice we have janitors and janitresses now in offices, and our young men unfortunately miss that salutary branch of business education. But if by chance the professional sweeper is absent any morning, the boy who has the genius of the future partner in him will not hesitate to try his hand at the broom. It does not hurt the newest comer to sweep out the office if necessary. I was one of those sweepers myself.
Assuming that you have all obtained employment and are fairly started, my advice to you is "aim high". I would not give a fig for the young man who does not already see himself the partner or the head of an important firm. Do not rest content for a moment in your thoughts as head clerk, or foreman, or general manager in any concern, no matter how extensive. Say to yourself, "My place is at the top." Be king in your dreams.
And here is the prime condition of success, the great secret: concentrate your energy, thought, and capital exclusively upon the business in which you are engaged. Having begun in one line, resolve to fight it out on that line, to lead in it, adopt every improvement, have the best machinery, and know the most about it.
What does the underlined word "it" in the last sentence refer to?
A.That line
B.The great secret
C.The best machinery
D.Every improvement