unassailable

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

"He successfully climbed the mountain."

"Some believe that there is undeniable evidence of UFOs."

"The castle walls were impregnable, so the invaders gave up trying to enter. "

"My arguments for a later curfew were based on my good grades and responsible behavior. "

"Muggers assailed them as they entered an alley."

"That he was the greatest 100m runner of his generation was incontestable."

"Her statement that Hitler was evil is incontrovertible."

"The contrary of a good report card is a bad one. "

"There is no evidence that anyone was here earlier."

"The library offered an extension on the due date for the overdue books. "

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