Was it in 1998,when he was still at middle school,------ thi

选C,强调句的疑问形式.
when he was still at middle school是定语从句,修饰1998
原句可以看成 Was it in 1998 （that） this boy became an expert at computer?
陈述句为It was in 1998 （that） this boy became an expert at compute