Verified answer

 a) some, any, no. 
 
Example: There were ... of my friends there. - There were some of my friends there. 
 
1. There were some of my friends there. 2. Well, anyway, there is no need to hurry, now that we have missed the train. 3. Have you ever seen any of these pictures before? 4. There is no water in the kettle: they have drunk it all. 5. There were some fir-trees in that forest, but many pines. 6. We could not buy cherries, so we bought some plums instead.
 
b) somebody, anybody, nobody. 
 
Example: I saw ... I knew at the lecture. - I saw nobody I knew at the lecture. 
 
1. I saw nobody I knew at the lecture. 2. I dare say that there may be somebody at the lecture that I know, but what does that matter? 3. Do you really think that anybody visits this place? 4. I have never seen anybody lace their boots like that. 
 
c) somewhere, anywhere, nowhere. 
 
Example: I haven't seen him ... . - I haven't seen him anywhere. 
 
1. I haven't seen him anywhere . 2. I know the place is somewhere about here, but exactly where, I don't know. 3. Did you go anywhere yesterday? - - No, I went nowhere , I stayed at home the whole day.