Protostars are difficult to observe because
a. the protostar stage is very short.
b. they are surrounded by cocoons of gas and dust.
c. they radiate mainly in the infrared.
d. the protostar stage is very short, they are surrounded by cocoons of gas and dust, and they radiate mainly in the infrared.
e. they are all so far away that the light hasn't reached us yet.
Ans: d. the protostar stage is very short, they are surrounded by cocoons of gas and dust, and they radiate mainly in the infrared.
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