When the reached the top of the hill, they _____ so they had to stop for a rest. [ ]

A. held their breath

B. out of breath

C. lost their breath

D. breathed deeply

氮素不足时，植株矮小，叶色浅淡。初期症状，先是新叶叶色失绿，变为浅绿或黄绿。