（2010·重庆模拟）—Alice, you look puzzled．Haven’t you understood it? —Yes．I _____of another problem just now． 

In the past several weeks, many cities in Northern China        from heavy fogs and  severe cold.

It was for the first time that the two countries ______ an agreement.

–Why do you know so much detail about the accident? --I ________ a walk nearby when it happened.

 Since last year I____ him only once.