—Every four years. The 32st Olympics will take place in Tokyo in 2020.
A:how far
B:how long
C:how much
D:how often
观察细胞中染色体行为并计数时,使用光学显微镜的正确方法是( )。
实现塔里木盆地农业可持续发展的有效措施是( )