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:

The Mexicali is the capital city of the Baja California and it has a population of about 689,775 and this city was historically agricultural-based and has an area of about 113.7 km square. As the early arrival of the Spanish in the place that led to the evangelization of this area in beginning, it was an important center of the cotton production. The Tijuana is also another largest city of the Baja peninsula and is the second largest and the rapidly growing in the terms of the Asian population and was a growing cultural center and has been recognized as a new cultural mecca.