Why is the gcf of two numbers sometimes one

When you have two primes, the gcf is 1. This is because a prime's onlyfactors are 1 and itself. Obviously two different primes cannot have a gcf of itself, so the gcf must be 1.

I’m pretty sure the answer is a

Planet earth has a mass of 5.9726x10²⁴ kg.then: 5.9726x10²⁴ kg=100% of the total earth mass.if the water´s mass=5x10-4 % of the total mass, we can solve this problem by the rule of three.5.9726x10²⁴ %*10⁻⁴ %x=(5.9726x10²³ kg)(5*10⁻⁴  %) / 100 %=2.9863x10¹⁹ kganswer:   the mass of the planet´s water would be   2.9863 x 10¹⁹ kg.