A 4 $\mathrm{kg}$ box is accelerated upwards by a string with a breaking strength of 80 $\mathrm{N}$ . What is the maximum upward acceleration that can be applied on the box without breaking the string?
(A) 2.5 $\mathrm{m} / \mathrm{s}^{2}$
(B) 5.0 $\mathrm{m} / \mathrm{s}^{2}$
(C) 7.5 $\mathrm{m} / \mathrm{s}^{2}$
(D) 10 $\mathrm{m} / \mathrm{s}^{2}$