– Oh, God! I’ve made such a mistake! –()(A) Oh, nothing!(B) Never mind. We all make mistakes.(C) You should blame yourself.(D) Don’t worry.

–– Jenny (Treading on someone’s foot):____I hope I haven’t hurt you.

–– John: It ’s all right.

A: Oh, I ’m sorry.

B: Oh, I ’m regretful.

C: Oh, excuse me.

D: Oh, forgive me.

I ()supper when I()someone knocking at the door.

A、made . . . Heard

B、made . . .was heard

C、was making . . . was hearing

D、was making . . . heard

B 宽度优先（种子染色法）

5．关键路径

几个定义： 顶点1为源点，n为汇点。

a. 顶点事件最早发生时间Ve[j], Ve [j] = max{ Ve [j] + w[I,j] },其中Ve (1) = 0;

b. 顶点事件最晚发生时间 Vl[j], Vl [j] = min{ Vl[j] – w[I,j] },其中 Vl(n) = Ve(n);

c. 边活动最早开始时间 Ee[I], 若边I由<j,k>表示，则Ee[I] = Ve[j];

d. 边活动最晚开始时间 El[I], 若边I由<j,k>表示，则El[I] = Vl[k] – w[j,k];

若 Ee[j] = El[j] ，则活动j为关键活动，由关键活动组成的路径为关键路径。

求解方法：

a. 从源点起topsort,判断是否有回路并计算Ve;

I've _____ these lines again and again but I still keep forgetting them.

A. gone off

B. gone about

C. gone over

D. gone through

– Are you sure about that?– ( ).

A. You needn't worry about that

B. I like the idea

C. Oh, no. I'm afraid of that

D. Oh, yes. I'm absolutely positive