Liu Yong, the first group of college students volunteering to serve the west, wrote a sentence in his diary: "There is a kind of work, and you do nt know the hardships before you experience it; there is a kind of hardship, you do nt know the happiness of it before you experience it; There is a joy, without knowing the purity of it. ". This paragraph Tian mainly expressed? ['Like is the foundation of happiness.', 'The true meaning of happy work methods.', 'Happiness comes from your sincere commitment to work.', 'No matter what work, hardship, difficulty, and distress exist objectively.']