English Grammar 高手請進

1 should be a.
The agreement on Hong Kong's future was signed a year before.
The agreement on Hong Kong's future had been signed for a year.

a year before says when (was signed when)
for a year says how long (had been signed for how long)

2 could be a
which was put on the table
which had been put on the table

I put it on the table (you sure that it was on the table, and should be there)
I had put it on the table (you sure that it was on the table, and only that)

So a is not wrong for both.

因為你那兩句句子的連接都有先後次序和指定發生時，一件事發生在前而跟著才有另一事．

香港前途的agreenment 已經在一年前簽了，a year befor. 之後的那句你說HK seemed a place to do business,　這句話好像有點問題，why not to write: HK seemed a GOOD place to do business. 所有地方是可以做生意，在乎好不好，適不適宜做生意．

第二句也是因為不見了那件已經放在門旁枱上的行李，如你沒先放，那又怎會不見行李?

1) (a) is wrong because "before" use past particple , not past tense
2) (a) is wrong because you use past particple before the past tense"could" which mention earlier time

1a
2b