What are possible reasons for this system message?

A user creates an incoming payment for a customer and selects two invoices to be paid. When the user tries to post the payment the system message that is shown in the attached screenshot appears.

What are possible reasons for this system message? Note There are 2 connect answers to this question.
A . The user did NOT choose a payment means
B . The amount in the Payment Means window does NOT match the total amount of invoices
C . The customer uses a foreign currency and the exchange rate is NOT set up
D . The invoices are NOT due yet

Answer: A,B