En cuanto/apenas/tan pronto como

Question

I am confused about these. The lesson says they are interchangeable, but when I do the quizzes I get marked incorrect for choosing - for example - apenas instead of en cuanto. Can anyone help clarify this?

Answer

Hola Anita
They are interchangeable but you also need to see whether the different options are using the indicative or the subjunctive. For example, here: 
Señora, la atenderemos__________ con este cliente.
Madam, we'll be with you as soon as we finish with this customer. 
tan pronto como terminemos
apenas terminamos
en cuanto terminamos
The only correct answer here is the first one, which is using the conjunction with the verb in subjunctive as this is expressing a future action. 
There is a note in the lesson about this;
I hope it clarifies it. 
Saludos