问题补充：
These flowers should not be watered too , once .
A. long; every other day
B. often; every fourth day
C. soon; every two days
D. frequently; each five days
答案：
A. long; every other day
(答案→)B. often; every fourth day
C. soon; every two days
D. frequently; each five days
解析：第一空需要副词，修饰动词water, 排除A项；从题意看，浇水不要太勤，排除C项；each后接单数动词，排除D项。every fourth day此处相当于every four days. 故B项正确。
