Under which condition can lake water become less useful for agriculture?

Correct answer: When salinity is very high

Direct answer: A, when salinity is very high. Irrigation water must have suitable quality as well as adequate quantity. If water contains too much dissolved salt, repeated use can add salts to the soil. This may make it harder for plant roots to take up water and can reduce crop growth, depending on the crop and soil conditions. Option A is correct because excessive salinity can make lake water unsuitable or less suitable for agriculture. Option B is wrong because clean water is generally more useful, not less useful, provided its salt content is suitable. Option C is wrong because sufficient quantity helps irrigation; it does not itself make water harmful. Option D is wrong because controlled outflow does not automatically make water saline. Memory cue: “For irrigation, remember both quantity and quality; too much salt harms crops.”