下列几种细胞在进行细胞分裂时,始终观察不到染色体的是①洋葱根尖分生区细胞 ②变形虫细胞 ③大肠杆菌细胞 ④蛙的红细胞⑤人的造血干细胞
It is cold in the room. They ______ have turned off the heating.