(a)[1]
Complete the sentence: Protostars are produced from [BLANK].
(b)[2]
Complete the sentence: A protostar becomes a stable star once [BLANK] and [BLANK] are balanced.
(c)[1]
Complete the sentence: The initial fuel that powers nuclear reactions in stars is [BLANK].
(d)[1]
Complete the sentence: Stars that are approximately the same size as the Sun become red giant stars and then produce a [BLANK] with a white dwarf star at its centre.