Choose either the present simple, present progressive, perfect or past simple: it (get) dark. let's go home; i (write) the letter but i can't find a stamp; can you see who (sit) in the park?

It gets dark. Let's go home.
I have wrote a letter but I can't find a stamp.
Can you see who sits in the park?

It is getting dark.
I have written the letter...
...who is sitting