10 Many tourists who come to Britain visit capital city because London is so famous. *(a/the)
11 My brother played a practical joke on me while I *(was sleeping/had slept)

10. Many tourists who come to Britain visit a capital city because London is so famous.

11. My brother played a practical joke on me while I was sleeping

Объяснение:

10. The
11. Was sleeping
Правила: 1. Если говорим о неизвестном предмете, то неопределенный артикль A / An. Если речь идет о чем-то определенном, то перед ним ставится артикль The.
2.i/you/we/they+was/were+VERB+ing