阅读下面短文，从短文后各题所给的四个选项（A、B、C和D）中选出可以填入空白处的最佳选项。
No doubt we all have experienced frustration（受挫）. It may be something as 1 as growing flowers or as serious as developing better relationships with others. Whatever the 2 is, there comes a point when you just feel you can't go on any further.
And yet, the secret to 3 can be found in a tube of toothpaste. Every time I come to the 4 of a tube of a toothpaste, I am completely 5 that just when I think it is absolutely empty, I can 6more toothpaste out of it. I am sure you all have gone through the 7 thing.
So, next time you find yourself in such a situation, 8 the tube. We can all 9times that success came when we were on our 10 chance or when we were doing something for the final time. It is when you dig deep down for that one last burst of 11 that you achieve your goal.
12 , too many people give up too early. All they 13 to do is squeeze one more drop of toothpaste from the tube. There are hundreds of 14 of successful people who kept on going in the face of adversity(逆境) and 15. Even the writers of Chicken Soup for the Soul were 16 by 33 publishers before they found one who would 17their book.
If you have the passion to go for your 18, then keep squeezing that “inner tube of toothpaste”. It is only when you 19 the “passion” that you will know whether success is yours or not. Just make sure that you don't 20 too soon. Success may be just around the corner.
