Tetraboric Acid, H₂B₄O₇, Is Prepared by Heating Boric Acid, H₃BO₃

Tetraboric acid, H₂B₄O₇, is prepared by heating boric acid, H₃BO₃ (a condensation reaction involving water loss) . If 400 mmol H₃BO₃ are used, what mass (g) of H₂O is formed, assuming quantitative stoichiometric conversion ?

A) 7.21
B) 9.01
C) 0.320
D) 0.500
E) 5.77