单选题：[1] As I sat perched in the second-floor window of our brick

[1] As I sat perched in the second-floor window of our brick schoolhouse that afternoon, my heart began to sink further with each passing car. This was a day I'd looked forward to for weeks: Miss Pace's fourth-grade，end-of-the-year party. Miss Pace had kept a running countdown on the blackboard all that week, and our class of nine-year-olds had bordered on rebellion by the time the much-anticipated “party Friday” had arrived. ①
[2] I had happily volunteered my mother when Miss Pace requested cookie volunteers. Mom's chocolate chips reigned supreme on our block, and I knew they'd be a hit with my classmates. But two o'clock passed，and there was no sign of her. Most of the other mothers had already come and gone, dropping off their offerings of punch, crackers, cupcakes and brownies. My mother was missing in action.
[3]“ Don't worry, Robbie, she'll be along soon,” Miss Pace said as I gazed forlornly down at the street．I looked at the wall clock just in time to see its black minute hand shift to half-past.
[4] Around me, the noisy party raged on, but I wouldn't leave my window watch post. Miss Pace did her best to coax me away, but I just stayed there, holding out hope that the familiar family car would round the corner, carrying my rightfully embarrassed mother with a tin of her famous cookies tucked under her arm. ②
[5] The three o'clock bell soon jolted me from my thoughts and I gloomily grabbed my book bag from my desk and shuffled out the door for home.
[6] On the walk to home, I plotted my revenge. I would slam the front door upon entering, refuse to return her hug when she rushed over to me, and vow never to speak to her again. The house was empty when I arrived and I looked for a note on the refrigerator that might explain my mother's absence, but found none. My chin quivered with a mixture of heartbreak and rage. For the first time in my life, my mother had let me down.
[7] I was lying face-down on my bed upstairs when I heard her come through the front door.
[8] “Robbie,”she called out a bit urgently. “Where are you?”
[9] When she entered my room and sat beside me on my bed, I didn't move but instead stared blankly into my pillow refusing to acknowledge her presence.③
[10] “I'm so sorry, honey,” she said. “I just forgot. I got busy and forgot—plain and simple. ”
[11] I still didn't move.
[12] My mother began to cry. “I'm so sorry,” she sobbed. “I let you down. I let my little boy down. ”
[13] She sank down on the bed and began to weep like a little girl. I was dumbstruck. I had never seen my mother cry. To my understanding, mothers weren't supposed to.
[14] I desperately tried to recall her own soothing words from times past when I'd skinned knees or stubbed toes, times when she knew just the right thing to say.
[15] “It's OK, Mom,”I stammered as I reached out and gently stroked her hair. “We didn't even need those cookies. There was plenty of stuff to eat. Don't cry. It's all right. Really. ” <