选C.请看解析：
首先,做此类题要学会分析句子,因为句子很长.实际上,这是三个句子合成的,
I don't remeber,
how many years ago was it?
I had dinner with you in that restaurant.
其次,how many years ago was it接在I don't remeber后面作宾语从句,要用陈述语气,所以合在一起后就变成how many years ago it was.排除A、D
再看,I don't remeber how many years ago it was 要和I had dinner with you in that restaurant.连起来,就要选个连接的词,因为在I had dinner with you in that restaurant中,主谓宾成分齐全,所以选用时间副词when作时间状语连接.that不行,因为它要在句子中充当成分,所以排除B,选C.
综述,选c.
本人为英语专业生,大四.希望对你有用.欢迎提问.