As part of its efforts to prevent manipulation of the platform by bots and fake accounts, Twitter challenged 19.35 crore accounts for verification in the second half of 2018, 17 percent fewer than the first half of last year, according to a transparency report from the micro-blogging site.
Twitter uses the "anti-spam challenge" for confirming whether a human is in control of an account.
Twitter may require the account holder to verify a phone number or email address, or complete a reCAPTCHA.
These challenges are simple for authentic account owners to solve, but could be difficult (or costly) for spammy or malicious account owners to complete.
"Accounts which fail to complete a challenge within a specified period of time may be suspended," said the report.