What a bore school is nowadays, the same as it has been for hundreds of years. What we get is the same old thing: teachers, outdated textbooks, and a class fed up to the teeth with the teacher and the school. What we need is one great change in the educational system of the country.

此句中的one great change，为什们不用a great change?

The young woman glanced up at his face frankly, took the envelope, and turned it curiously in her hand. "Ralph Matthews," she repeated the name as if it sounded strange to her, "I don't think I know him." Nevertheless, she opened the envelope and took out the paper. "Why, it's a blank sheet!" she remarked, puzzled.

The detective turned suddenly to Mr. Grayson who had looked on with frank astonishment. "May I use a telephone, please?" asked The Thinking Machine.

如何理解frank astonishment中的frank的意思？