a keen observer的用法和样例：
- She is a keen observer, and never flinched from pointing out the faults.
- Here is the source where the artist finds its inspiration and has proofed himself a keen observer.
- A keen observer of English customs relates that, being in one of the rooms of the Bank one day, he had the curiosity to examine a gold ingot weighing some seven or eight pounds.
- She is a keen observer of even the cruelest workaday details, as when Nini is hungry enough to eat the flour paste being used to put up posters about Gu Shan’s imminent execution.
- The blind have a keen sense of touch.