A rich man loved his little boy very much. He wanted 1  him happy all the time. So he gave him everything that he could buy with   2 . But the little boy was still  3  .Where he went, he always wore a frown(皱眉). And he always wished for something he did not have. The rich man didn't know   4  to make his son happy.
    One day, a magician(魔术师) came and said to him, “I can make your son happy and  5  his frowns into smiles, but you must pay me a great price   6 telling him the secret.”
   “All right,” said the man, “No matter what you ask for, I  7  you.”The magician took the boy into a secret room. He wrote something on a piece of paper, and then gave it to the boy. There were some words on the paper, “Do one  8  thing for someone every day.” The boy followed the  9   and became one of the happiest boys.
    I think only those who are always thinking about   10  can be truly happy.

i是虚数单位，复数$\frac{5+3i}{4-i}$=（　　）