Can someone plese help me with these two science questions?

Question

One half of the radioactive potassium -40 (K^40) in an igneous rock has decayed to argon -40 (Ar^40) About how many years ago was this rock formed?

1. 1.3x10^9

2. 2.6x10^9

3. 3.9x10^9

4. 9.8x10^9

Uranium-238 is used to date the age of Earth rather that carbon-14 because uranium-238

1. was more abundant when Earth formed

2. has a longer half-life

3. decays at a constant rate

4. is easier to collect and test.

Answer 1

Don't know the answer to the first question, bacause I can't do maths, but for the second I think the answer is 2. I think it has something to do with there not being enough carbon atoms to test after a certain period of time.