Assertion: In water, the ratio of mass of hydrogen to the mass of oxygen is always 1:8 whatever the source of water.
Reason: According to the law of constant proportion, the elements are always present in definite proportion by mass in a chemical substance.

A. Both assertion and reason are true and the reason is the correct explanation of the assertion
B. Both assertion and reason are true but the reason is not the correct explanation of the assertion
C. Assertion is true but the reason is false
D. Assertion is false but the reason is true