Explain the following statements.

a. A car exhaust pipe will rust much faster if the car is in constant use.

b. Vegetables cook faster when they are chopped up.

c. Industrial processes become more economically viable if a catalyst can be found for the reactions involved.

d. In fireworks it is usual for the ingredients to be powdered.

e. Tomatoes ripen faster in a greenhouse. 

f. The reaction between zinc and dilute hydrochloric acid is slower than the reaction between zinc and concentrated hydrochloric acid.