Tijuana and Mexicali:

a. are key cities in the south of Mexico.
b. are planning to participate in the maquiladora program.
c. are located just opposite the U. S. city of El Paso.
d. were among the first border cities to experience a growth surge.

d. were among the first border cities to experience a growth surge.

Explanation:

answer: d.

a lack of valuable natural resources

explanation:

d. stage 4

explanation:

the existence of some kind of demographic transition is widely accepted in the social sciences because of the well-established historical correlation linking dropping fertility to social and economic development.[1] scholars debate whether industrialization and higher incomes lead to lower population, or whether lower populations lead to industrialization and higher incomes. scholars also debate to what extent various proposed and sometimes inter-related factors such as higher per capita income,lower mortality, old-age security, and rise of demand for human capital are involved.[2]