What does the author mean by saying "more challenging" in the last paragraph?
______first made Mark Twain known all over the country.
Psychemedics Corp is a company that______drug test kits.
When must the sonar devices be placed?
Before the ______.
Stars of television and radio who score high on the likability scale have ________.
The parents stared at their child angrily,______(等着他来解释事情的原委).