阅读理解
A wealthy man asked an old scholar to tell his son away from his bad habits. The scholar took the young man for a walk through a garden. Stopping suddenly, he told the boy to pull out a tiny plant growing there.
The young man made it easily. The old man then asked him to pull out a little bigger plant. The young man pulled hard and the plant came out with its roots. “Now pull out that one,” said the old man, pointing to a little tree. The boy had to use all his strength to pull it out.
“Now take this one out,” said the old man, pointing at a big orange tree. The young man held the trunk (树干) and tried to pull it out. But it would not move. “It's impossible”, said the boy, panting (喘息) with the effort.
“So it is with bad habits,” said the old man. “When they are young, it is easy to pull them out. But when they go deeply, they cannot be uprooted.”
The conversation with the old man changed the boy's life.
，则这个三角形一定是（ ）
