题目

—Look at the bird over there! It's so beautiful!—It's an unusual kind. It ____ appears here. A . always B . usually C . seldom D . often 答案：C在脑和脊髓中，神经元的 集中的部分，色泽灰暗、称为 ； 集中的部分，色泽亮白，称为 ．