Which strategy has been implemented to reduce Mexico City's air pollution? 

A)  planting 25 million trees 
B)  requiring the removal of catalytic converters from cars 
C)  requiring all cars to use biofuels 
D)  tax incentives to improve energy efficiency

Planting 25 million trees is a strategy that has been implemented to reduce Mexico City's air pollution. It helps to decrease the concentration of pollutants in the air by absorbing carbon dioxide from the atmosphere. This reduces the greenhouse gas effect and helps mitigate the effects of climate change. The other options either do not have a direct impact on air pollution or could potentially make it worse.