A. Dynamic memory works by assigning an initial amount of memory and then adding additional memory if the VM is under memory pressure. Applications need to be able to take advantage of this additional memory as it's added, which isn't the case with Exchange 2010 Mailbox server role. This role checks the amount of memory present when the mailbox server starts and then doesn't recheck, so it won't take advantage of additional memory if it's added to the VM after the mailbox service has started.
Linux VMs can't use dynamic memory today because they don't support the hot addition of memory—so there's no way to give the Linux OS more memory while it's running. However, this may change in the future.