根据句意及首字母或汉语提示完成单词，使句子的意思完整、正确。

【1】Our new school c______ an area of 4.5 square kilometres.

【2】We bought a lot of souvenir(纪念品) during the t______ last month.

【3】It was so cold this morning that some ice______(形成)on the window.

【4】Who ______(介绍)one of his best friends to us just now?

【5】The new bike under the tree is the______ (主席) .