Dear Amy,

    Thanks for your letter. It's always nice to hear from you.

    Today is the last day of my exams. The maths exam is very difficult, but other exams are easy. I'mwaiting for the holidays now, and I'm very anxious to see you and to be with you again. I'm going to spend my Christmas and New Year holidays in New York with my sister. I'm going down town. I'm going to go shopping for Christmas gifts. I'm excited about Christmas, and about seeing you again.

                                                                                                                                                                                                        Yours,

                                                                                                                                                                                                           Jack

若3a=2b，则$\frac{a-b}{a}$的值为（　　）

花粉从雄蕊散发出来，落到柱头上的过程叫做（　　）

如图所示，对一植物的侧枝枝条树皮进行环割处理，一段时间后该枝条的切口处出现一个瘤状物，该瘤状物的形成是由于____堆积引起的，瘤状物在切口的____（“上”或“下”）方．