1. 主动语态与被动语态相互转换，每空一词（含缩写）。
56 . We call maths the language of science. （改为被动句）
Maths _________ _________ the language of science.
57. Where did they plant t rees? （改为被动句）
Where _________ trees _________?
58. You can borrow two books at a time.（改为被动句）
Two books _________ _________ _________ at a time.
