Classify each statement below as an observation or as an inference based on the current big bang model. (note that the helium abundance is defined as the mass of helium relative to the mass of hydrogen.)
1. photons of the cosmic microwave background have traveled through space for almost 14 billion years.
2. the cosmic microwave background is radiation left over from the big bang.
3. large-scale structures look about the same in all directions.
4. the temperature of the cosmic microwave background varies slightly with direction.
5. the cosmic microwave background temperature is 2.73 k.
6. the helium abundance is at least 25% in every galaxy studied so far.
7. fusion during the universe's first five minutes produced 75% hydrogen and 25% helium.
8. large-scale structure grew around density variations present in the early universe.

Observations:

While Inference includes:

Explanation:

Inference here refers to the process of drawing a conclusion based on what is already known while Observation involves the perception and recording of data via the use of scientific instruments and procedures.

The insights below highlights the explanations for some of the insights used in determining its Observation or Inference status.

The Cosmic Microwave Background, which contains the afterglow of light and radiation is left over from the Big bang.

The Cosmic Microwave Background is black body radiation at a temperature of 2.725 Kelvin.

In the year 1965, a pair of radio astronomers named Arno Penzias and Bob Wilson respectively were working at Bell labs, in New Jersey. Using an antenna that is sensitive to the radiation of microwaves, they discovered an unknown source of static noise. After they tried doing everything possible  to reduce the noise, they realized that the microwaves had a black body spectrum, with a characteristic temperature of only T = 2.725 Kelvin ( and maximum wavelength of 1 millimeter). These ever present microwaves seen in every direction are called "Cosmic Microwave Background". This explains the reason behind statement five(5) being an Observation.

Finally, The Cosmos Background Explorer (COBE) which launched a satellite in the year 1989 found that there were slight fluctuations in the radiation spectrum. The temperature varies very slightly from one place to another by a few parts in 100,000 i.e (10^-5). The excerpts above clarify the reason for the answer to statement four(4) being listed as an Observation.

real, smaller and upside-down.   so the answer should be b.