97. Yesterday we received a _____ in which several customers have found fault with our products.

A．letter of congratulations

B．letter of thanks

C．letter of criticism

D．commendatory letter

盐水选种是我国农民创造的一种巧妙的选种方法，在900多年前，就有了有关记载，现有一堆麦种，你如何用盐水选种法将好种子、坏种子区分开来？并说出你的理由？如果选出好种的比例过小，应怎么办？