Human vision can distinguish objects down to about a 4th of a millimeter (~250 micrometers). Given this, which of the following are generally visible to the human eye?

Proteins
Bacterial cell
Animal cell
Poxvirus
Ribosomes
Plant cell
None of the above

I think none of the above...

Plant cell

None of the above is right.

To determine which of the given options are generally visible to the human eye, we need to compare their sizes to the human eye's resolution limit of approximately 250 micrometers.

1. Proteins: Proteins are generally much smaller than the resolution limit of the human eye. They typically range in size from a few nanometers to tens of nanometers, which is significantly smaller than the 250 micrometer threshold. Therefore, proteins are not visible to the human eye.

2. Bacterial cell: Bacterial cells vary in size, but on average, they are around 1-10 micrometers in length. Since this falls within the range of what the human eye can typically distinguish, bacterial cells can be visible to the human eye under suitable conditions.

3. Animal cell: Animal cells are similar in size to bacterial cells, ranging from approximately 10-30 micrometers. As a result, they can generally be visible to the human eye given their size.

4. Poxvirus: Poxviruses are much smaller than the human eye's resolution limit. Their size typically ranges from 200-300 nanometers, which is significantly smaller than the 250 micrometer threshold. Therefore, poxviruses are not visible to the human eye.

5. Ribosomes: Ribosomes are also much smaller than the human eye's resolution limit. They have a size of around 20-30 nanometers, which again is smaller than the threshold. Therefore, ribosomes are not visible to the human eye.

6. Plant cell: Plant cells are similar in size to animal cells and bacterial cells, ranging from approximately 10-100 micrometers. Therefore, plant cells can generally be visible to the human eye given their size.

Based on this analysis, the options that are generally visible to the human eye are: Bacterial cell and Plant cell.