答案：A

解析：

Choice A is the best answer. After stating that Tom "claimed all her attention," the narrator says that Tom "stood bolt upright... talking away like an orator, occasionally interrupting his discourse with a sharp reproof to his sisters when they made too much noise." In this context, the words "orator" and "discourse" -- words associated with lecturing and speaking formally -- mainly emphasize that Tom is taking a position of authority, speaking to Miss Grey as if he is a teacher and expressing disapproval of his sisters' behavior as if they are his students.