過去完成式?!

我想問幾題我做的題目In the last fifty years

medical scientists____many important discoveries.a.made b.make c.have madei chose A. but the correct answer is C. Sam_____a newspaper reporter before he____a businessman.a.had been

became b.was

had become c.was

becamei chose A which is the right answer. but i think c is ok too. I______a little bitter after I____the medicine.a.felt

had taken b.felt

took c.had felt

tooki chose B. and A is correct. I was late. The teacher______a quiz when I_______to the class.a.gave

got b.had given

got c.has given

geti chose A. and B is correct.我知道過去完成式要用在比過去式更早發生的時候我也知道吃藥比較早發生所以要用過去完成式可是我一直以來before after都用簡單式耶是我錯了十幾年嗎???我總覺得都可以......常常聽到人家說 I went to America after I graduated from college.這樣是錯的嗎???????可是也有看過一句話He went out before I had fisnished my sentence.可是這裡是went out先發生拜託大家跟我解釋一下我做的題目 我的答案就一定是錯的嗎?
你沒有做錯!!!老外也是這樣用的我特別看了一下文法權威書Azar的註解與例句簡單來說

過去完成式只是要表達過去事件與時間點的先後順序如果句子中有before或after

因時間關係已經很清楚了

所以根本不需要使用過去完成式

例如以下兩句意義完全相同：1.Sam had left before Ann got here.(Sam先離開

Ann後到達)2.Sam left before Ann got here.(Sam先離開

Ann後到達)*比較: Sam left when Ann got there.(Ann先到達

Sam才離開)這樣放心了吧^^