【名词&注释】
维生素A(vitamin a)、缺铁性贫血(iron deficiency anemia)、三级预防(three-grade prevention)、一级预防(primary prevention)、佝偻病(rickets)、脚气病(beriberi)、坏血病(scurvy)、夜盲症(nyctalopia)
[单选题]()is the most visible signal of distress from a lifeboat at sea in the daytime.
A. Smoke signal
B. Flash signal
C. Fire signal
D. Radio emergency transmissio