According to the passage, which of the following posed the biggest obstacle to the development of military forts

A．By registering annual birth and death rates.
B．By experiments different building materials.
C．By maintaining records of diseases and potential causes.
D．By monitoring the soldiers' diets.

A．agree that the calories in a small piece of pastry can be difficult to work off by exercise
B．partially believe diet plays a supporting role in weight reduction
C．are not fully convinced that dieting can help maintain one's new weight
D．are not sufficiently informed of the positive role of exercise in losing weight