Final answer:
While the exact regions with the greatest concentration of JATC training centers are not given, historical context implies that regions like the West Coast and others with significant military presence during WWII likely have a higher concentration of such centers.
Step-by-step explanation:
The two regions of the U.S. with the greatest concentration of JATC (Joint Apprenticeship and Training Committee) training centers are not explicitly mentioned, but one can infer from the context provided that the question may be referring to regions that experienced significant growth during World War II. The data provided speaks to the rapid expansion of West Coast cities and towns throughout America due to military activity. While the provided information does not directly address JATC training centers, it is known that large cities and regions with significant industrial and military growth often have more vocational and training opportunities. Therefore, regions like the West Coast, with cities such as San Diego and those around military bases and industries, likely have a higher concentration of these centers, as do other regions that saw similar growth, such as the South and Midwest, where the military presence was also strong during that time.