Which of the following sentences does not properly use a subordinating conjunction

2 answers

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hers the choices
a.Because the closest star is so far away, humans will probably never travel there.

b.Many people believe that there is undiscovered life on other planets in our
galaxy.

c.There are billions of stars in our galaxy, and our galaxy is only one of millions.

d.Scientists continue to listen for messages sent by intelligent life on other planets. @electroShark797 answers plz