They fell in love with each other the first time () they met

They fell in love with each other the first time () they met at a birthday party.1)不填 2) at which
1)不填 2) at which 3)that 4）when 四个选项哪些是正确的,并能说明一下选择的理由,
应不止一个选项是正确的。但大家都选了一个，难道真的只有一个选项是正确的。