Where does the air come from when the cross bleed valve is selected from the left?

When the cross bleed valve is selected from the left, the air source comes from the left engine. This is because the purpose of the cross bleed valve is to allow airflow from one engine to another or to another system, thereby facilitating engine start procedures or providing bleed air for cabin pressurization and air conditioning.

In this scenario, selecting from the left indicates that the left engine is supplying the necessary bleed air. This is particularly useful when one engine is inoperative or when additional airflow is required for specific operations. The other options encompass air sources that are not applicable when selecting from the left, such as the right engine, the APU (Auxiliary Power Unit), or external air supplies. Each of these would function in different contexts but is not relevant to the operation when the cross bleed valve is selected from the left engine.