In the following questions, sentences are given with blanks to be filled with an appropriate word(s). Four alternatives are suggested for each question. Choose the correct alternative out of the four as your answer.

Serious threat to our ecology and environment can be ______ with organic cultivation.

A. hastened

B. impeded

C. aggravated

D. combated

Please do not use chat terms. Example: avoid using "grt" instead of "great".