Here are the best choices to complete each sentence:

85. **A. don’t know / was**
86. **C. sent/sent**
87. **A. was writing**
88. **B. Have/seen**
89. **C. did**
90. **A. is studying**

So, the completed sentences would read:

85. I **don’t know / was** how to dance when I was six years old.
86. Last month my brother **sent** me his photos. He **sends** me photos every year.
87. While her husband was in the army, Janet **was writing** to him twice a week.
88. **Have** you **seen** that film yet?
89. While mum was watching TV, I **did** my homework.
90. Anita **is studying** very hard at the moment.