When some solid barium hydroxide and solid ammonium thiosulfate were reacted together, the temperature of the surroundings was observed to decrease from 15 °C to –4 °C. What can be deduced from this observation?

A.     The reaction is exothermic and $\Delta H$ is negative.

B.     The reaction is exothermic and $\Delta H$ is positive.

C.     The reaction is endothermic and $\Delta H$ is negative.

D.     The reaction is endothermic and $\Delta H$ is positive.

D