Critical Reasoning

In the early 1900s, airborne asbestos particles were found to be carcinogenic, so a regulation prohibiting the construction of any residential building in which any airborne asbestos particles could be detected was put into effect by the early 1930s. However, to insulate many buildings, many construction companies continued to use asbestos extensively in residential construction into the early 1970s. The original regulation will soon be replaced by one that allows residency in buildings in which trace quantities of airborne asbestos particles are detected; however, this change will not allow more asbestos particles to be airborne in residential buildings than the number that were airborne in the 1930s, because ________________.

(A)before 1940, techniques for detecting the number of airborne asbestos particles could detect such particles only when they were present in buildings in amounts greater than the trace quantities allowed by the new regulations correct
(B)the number of people who move into a building without checking for acceptable asbestos compliance is higher today than it was in the late 1930s
(C)buildings constructed in the future under the new regulation will contain no more residents, on average, than buildings that were constructed in the late 1930s when the old regulation was in effect your answer
(D)there have been at least a small number of buildings for which the number of airborne asbestos particles exceeds the regulated limit every year since the late 1930s
(E)the presence of airborne asbestos particles in levels higher than those mandated by the upcoming regulation has not been shown to cause cancer, even in long-term building residents

IMO A
what's the OA?