pollute success   belong to   return to   three   mean mind

71．Trees can help protect the environment in many ways，because they are 　　fighters．

72．The Shenzhou XI spaceship returned to the Earth 　　．

73．This beautiful skirt 　　my sister，my mother bought it for her 2 years ago．

74．Don't give up，even though we have failed twice．Let's have a 　　try．

75．The 　　of this sentence is not clear to me．