Fully ________ in looking after three children at home, she no longer has time to enjoy   various activities in the club.

A．attached

B．occupied

C．contributed

D．devoted

有12袋白糖，其中11袋的质量相同，另一袋稍轻一点，如果用天平称，至少称几次可以找出这袋较轻的白糖？