a sack of flour的用法和样例：
- Now, as to vegetable, give me a sack of potato.
- He hiked up a sack of grain on his shoulder.
- The poor thing has a figure like a sack of potatoes.
- A day or two before I sailed a man who owned a number of pearlers came to me and asked whether on my way I would stop at the island of Trebuchet and leave a sack of flour, another of rice, and some magazines for the hermit who lived there.
- There is a sack of potatoes at the corner of the room.