Question

the rhine, danube, and lena are threatened by
aggressive fish species consuming other fish.
annual freezing and flooding.
shrinkage due to over irrigation.
water pollution.

Explanation:

To determine the cause of the decline of the Blue, Danube, and Lena (assuming these refer to aquatic ecosystems or related water bodies), we analyze each option:

• "aggressive fish species consuming other fish" would impact fish populations but not the overall ecosystem decline of the water bodies themselves.
• "annual freezing and thawing" is a natural cycle and not a cause of decline.
• "shrinkage due to over - irrigation" is more relevant to smaller water bodies or aquifers used for irrigation, not major rivers like the Danube and Lena (assuming the Blue refers to a water body affected by over - irrigation).
• "water pollution" is a major threat to water bodies, affecting aquatic life, water quality, and the overall health of the ecosystem, which can lead to the decline of these water bodies.

Answer:

The correct option is "water pollution".