Before revision:  The sources of multifractality are also investigated and it is confirmed that both temporal correlation and the fat-tailed distribution contribute to the multifractality, and the degree of multifractality for the time correlation is stronger than that for the fat-tailed distribution.
After revision:  The sources of multifractality are also investigated and it is confirmed that both temporal correlation and the fat-tailed distribution contribute to the multifractality, and the degree of multifractality for the temporal correlation is stronger than that for the fat-tailed distribution.