Comprehension Passage
Read the following passage:
The hydrogen wall has been described by scientists as the place ___(X)____ bubbles of solar wind cease to exist. The mass of interstellar matter here is very small ____(Y)_____ strong enough to not let the solar winds pass through. At the same time, it is not so strong to burst ____(Z)____ the solar wind build-up. It acts like a wall pressing the solar winds inward.
Select the most appropriate word from the options against _____(Y)______ .
1
since
2
because
3
but
4
in addition to