Which of the following is evidence that the soils of Mesopotamia became saltier? The farmers let more lands become fallow. The farmers switched from continuous irrigation to seasonal irrigation. The farmers switched from growing wheat to growing barley. The farmers switched from growing barley to growing wheat.

The **most direct evidence** that the soils of Mesopotamia became saltier is: **The farmers switched from growing wheat to growing barley.** **Explanation:** - **Barley vs. Wheat:** Barley is more **salt-tolerant** than wheat. When soils become salinized, farmers often switch to crops that can better withstand higher salt levels. This shift from wheat to barley indicates that salinity was becoming a significant issue affecting crop viability. - **Other Options:** - **Letting lands become fallow:** While this could be a consequence of soil salinization, it is not as direct an indicator as changing crop types. - **Switching irrigation methods:** Changing from continuous to seasonal irrigation might help mitigate salinization but is more of an agricultural adaptation rather than direct evidence of increased soil salinity. - **Switching from barley to wheat:** This would suggest decreasing salinity, which is the opposite of the scenario in question. **Conclusion:** The transition from growing wheat to barley is the strongest evidence among the options provided that Mesopotamian soils were becoming saltier. **Answer:** The farmers switched from growing wheat to growing barley.