Remember to minimize movement in your thumb. Since you will be using the whisper key throughout the etude, position your left thumb high on the key, close to the C# key. To play C#, slightly pivot your thumb upwards to press the C# and low D keys. You may also use the whisper key lock (if your instrument has one), so you do not have to press the whisper key at all, freeing your thumb to manage only the other two keys on C#.