Changes in the size of population , age and race
Answer:
1920s and 1930s,
Explanation:
it's worth reiterating that helmets were not mandatory. That wouldn't happen until 1943. During the 1920s and 1930s, variations of the leather helmet appeared, but in 1939, the game changed –or at least became safer– when John T. Riddel introduced the first plastic helmet
Answer:
because it gives common language for reporting and monitoring diseases
Im so confuse by this lol
explanation: what r u asking for to be exact..
