задание с фипи

[Gulnarochka]

]Здравствуйте. Проверьте пож-та:

 When I came into the room, my computer was on. It worried me since I _________NOT/REMEMBER_________ switching it on. I looked around –  there _________BE_________ nobody in my room and everything was in its usual place.  Suddenly I _________UNDERSTAND_________ –  the book had gone! I had left it on the table but it wasn’t there any more. I checked the window –  it _________CLOSE_________. That meant that someone had come into the room through the door. My _________ONE_________ impulse was to call the police but I called Jim instead. “What _________HAPPEN_________?” Jim sounded sleepy. I explained that someone _________TAKE_________ the book. “Oh, no!” Jim sounded unhappy. “Have you any idea how valuable it is?” “There’s another thing I ________NOT/CAN__________ understand,” I interrupted Jim. “My computer’s on but I always switch it off before leaving home.” “Ok, stay at home. I’ll be at _________YOU_________ place in half an hour,” he said and the phone went dead


Мои ответы:
didn't remember
was
understood
was closed
first
has happened
had taken
can't
your

особенно смущает первое предложение. вроде как есть since, но я думаю оно идет в значении "так как".
Спасибо