下列各句中，加横线的成语使用正确的一项是（ ）

Mr. Wang made up his mind to devote all he could______his oral English before going abroad .

The initial tunnels were just ___________ the surface of the ground. People were transported in carriages, which were ___________ the narrow tunnels by steam engines.

” is the hardest word to say sometimes. _______you know you’ve done something wrong, or something inappropriate, it takescourage to say sorry.

1—What’s your favoritesubject—________because I am interested in numbers.