flat submanifold

例句：

1. 1. The floor is flat and easy to clean.
   地板是平坦的，容易清洁。
2. 2. The table is flat, so it's easy to move around.
   桌子是平坦的，所以很容易移动。
3. 3. The road is flat, which makes driving easier.
   道路是平坦的，这使得驾驶更容易。