a. 心不在焉的, 健忘的

attention/attend/attentive/intend/intention/extend/extensive/pretend/absent-minded/notice
记忆：
1. 词根tend表示“倾向, 伸张”，attend vt. 走过去出席，参加，照顾 I will attend an important meeting tomorrow. 我明天要出席一个重要的会议。
2. 加名词后缀-ion, attention n. 心思在那里，注意，注意力 You need to pay attention to your pronunciation. 你需要注意你的发音。
3. 加形容词后缀-ive, attentive adj.专心的 We should be attentive in class. 课堂上我们要专心一点。
4. in 表示在里面， intend vt. 心思向里面伸张，即有此打算，想要 He intended to set up a company of his own. 他打算建立一家自己的公司。
5. 加名词后缀-ion, intention n. 打算，意图 His intention is obvious. 他的意图很明显。
6. 前缀ex-表示“向外”， extend 向外伸张，v.扩充, 延伸, 伸展, 扩大 With economy growing, the country plans to extend. 随着经济的发展，这个国家想要扩展。
7. 加形容词后缀-ive, extensive adj. 扩展的，延伸的 We need to do some extensive reading. 我们需要一些泛读练习。
8. 加前缀pre-事先，pretend vi. 假装 How could you pretend that you had never met me before ? 你怎能假装以前没见过我？
9. absent-minded adj.心不在焉的 As you were absent-minded, it’s no wonder that you know nothing about the speech. 你心不在焉的，难怪你对讲演一无所知。
10. notice vt. & n. 注意到（强调结果），通知（书面）He took no notice of the notice on the blackboard. 他没有注意到黑板上的通知。
另类阅读：
The teacher noticed Tom absent-minded in class, and intended to call his attention, “ Tom, please pay attention to what I will do.” Tom had no intention to follow his teacher, but he had to pretend to be attentive in class.