two pieces of 后面加单数还是复数

　　Glue the two pieces of cardboard together.

　　把这两张硬纸板粘在一起。

　　The two pieces of wood knocked together.

　　两块木头互相碰撞。

　　He sent her two pieces of olives in letter.

　　他在信中给她夹了两片橄榄树的叶子。

　　The two pieces of wood always jam together if I don't hold them apart.

　　我要是不把这两块木板撑开，它们就老是挤在一块。

　　There is a row of trees at the line of demarcation between the two pieces of land.

　　两块地之间有一排大树作为分界。