I'm very glad to write back to you _______I heard from you three years ago.A after B brfore C when D at that time

I'm very glad to write back to you _______I heard from you three years ago.A after B brfore C when D at that time
I'm very glad to write back to you _______I heard from you three years ago.
A after B brfore C when D at that time

I'm very glad to write back to you _______I heard from you three years ago.A after B brfore C when D at that time
A:
I'm very glad to write back to you after I heard from you three years ago.
三年前收到你的信后,我就很乐意给你回信.
Before不符合意境,when和at that time 要用过去时,不能用I'm very glad to