good at 與 good for 分別?

常見 good 是跟 at, 解擅長
You are so right!
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但點解選擇題例句中有例子


He is good for nothing. ( useless)
He is a good-for-nothing( noun). ( A useless male)
good for, a. certain to repay (money owed) because of integrity, financial stability, etc.
b. the equivalent in value of: Two thousand stamps are good for one coffeepot.
c. able to survive or continue functioning for (the length of time or the distance indicated): These tires are good for another 10,000 miles.
d. valid or in effect for (the length of time indicated): a license good for one year.
e. (used as an expression of approval): Good for you!


Definitions from dictionary.com
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In English, we say " good in " too.
Example:
He is good IN BED.<----------床上功夫好好
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經一些報道俄軍向中國貨船開火後，昨天中國請求俄羅斯解釋為何--中國貨船在其水域沉沒。
To call on XXX = 請求
after REPORTS<--------- It is a noun.一些 reports

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good at是”善於”之意，如：
He is good at swimming（就是說他游泳了得）
至於good for就是”對．．．有益”，如：
It is good for you to take a rest.（休息一下對你有益）
至於good for nothing的意思，是”useless”，即無用之意

而
China called on Russia yesterday to explain how a Chinese cargo ship sank in Russian waters after reports it was fired on by the Russian military.
明顯after在這裏是作”介詞”(preposition)用，當然reports顯然是名詞了，這裏作”眾多報導”
或許你將句子這樣寫
China called on Russia yesterday to explain how a Chinese cargo ship sank in Russian waters after reports, it was fired on by the Russian military.
（在眾多報導後，中國要求俄羅斯解釋中國貨船如何在俄海域沉下，這船被俄軍擊中）

希望幫到你！



2009-02-20 10:57:24 補充：
擊中改譯為”砲轟”更好

2009-02-20 11:00:08 補充：
對不起，表達不大清楚
或許將句子看為這樣更好
China called on Russia yesterday to explain how a Chinese cargo ship sank in Russian waters after reports　ｔｈａｔ it was fired on by the Russian military.

2009-02-20 11:02:20 補充：
（在眾多報導貨船被俄軍砲轟後，中國要求俄羅斯解釋中國貨船如何在俄海域沉下）

2009-02-20 17:02:31 補充：
忘記了解決call on，call on除解作”探訪”外

call on someone [ to infinitive]，其意是ask someone in formal way to do something即”要求”之意

以下 Cambridge Online Dictionary 解釋得很清楚：

good-for-nothing
noun [C] INFORMAL
a worthless person:
She told him he was a lazy good-for-nothing and should get a job.

good-for-nothing
adjective [before noun]
useless:
They're all good-for-nothing layabouts.

call on sb (ASK) phrasal verb [+ to infinitive]
to ask someone in a formal way to do something:
They're calling on all men and boys over the age of 14 to join the army.
FORMAL I now call on everyone to raise a glass to the happy couple.

至於 reports，是指中國報道這事件，是動詞，全句意思是：中國報道俄軍向中國貨船開火後，昨天要求俄羅斯解釋中國貨船如何在俄羅斯水域沉沒。

good at 解作技能上精於某方面
good for 解作有效期