完成对话

A：Hello？（71）　　？

B：Hi，Lisa．It's Anna here．

A：Oh，hi，Anna．（72）　　？

B：I am studying for the math test．

A：I called you at 4：30this afternoon and you didn't pick up．（73）　　？

B：I was at the city park with my sister at that time．

A：（74）．　　？

B：We were picking up litter near the lake．

A：I'd like to join you，too．Can I go with you next time？

B：（75）．　　．