Official websites use .gov
A
.gov website belongs to an official
government organization in the United States.
Secure .gov websites use HTTPS
A lock (
) or https:// means you've safely
connected to the .gov website. Share sensitive
information only on official, secure websites.
(A) The keyword occurrence dynamic from 2000 to 2022. (B) The evolution of keywords over three time periods based on their frequency of occurrences. (C) Keyword co-occurrence map of 50 words: The three clusters were in red, blue, and green, respectively.
