Answer: Both Arianna and Alexander
Explanation: This is a general partnership. In a general partnership, all partners are personally liable for all business debts. They do not need to have any agreement to be partners or register their partnership formally to enter into a general partnership. The fact that they share an office, have a joint sign and an account is sufficient to establish this form of partnership.
Also, in a general partnership, each partner is held severally liable, that is if one of them is liable to pay a business debt and cannot afford to pay, the other partner has to.