challenging

"My friend told me there was something in the box, but I didn't know what it was yet. "

"The competition for this job is strong."

"We challenged the boys next door to a game of football."

"The historical romance novel was a bestseller in the bookstore. "

"Attendance at the meeting is required."

"This flow chart represents the assignment of tasks in our committee."

"The application required a countersign from the department head to validate the principal's signature. "

"He completed the assignment on time."

"The teacher threw a challenge to the students: "Who can solve this difficult math problem?" "

"My discussion with the professor was very enlightening."

"My teacher made a statement about the importance of studying hard. "

"His assumption of secretarial duties was timely."