Questions: Viruses are usually nanometers in size, and their structure is best viewed through the use of electron microscopy. Multiple Choice nanometers; electron micrometers; electron micrometers; bright-field nanometers; bright-field

The answer is the first one: nanometers; electron.

Explanation for each option:

1. nanometers; electron: This is correct. Viruses are typically measured in nanometers (nm), which are one-billionth of a meter. Their small size makes them difficult to see with standard light microscopes, so electron microscopy, which has much higher resolution, is used to view their structure.

2. micrometers; electron: This is incorrect. Micrometers (µm) are one-millionth of a meter and are used to measure larger entities like cells and bacteria, not viruses. While electron microscopy is indeed used to view viruses, the size unit here is incorrect.

3. micrometers; bright-field: This is incorrect. Bright-field microscopy is a common type of light microscopy used for viewing larger specimens like cells and bacteria, which are measured in micrometers. However, it does not have the resolution needed to view viruses, which are much smaller.

4. nanometers; bright-field: This is incorrect. While viruses are measured in nanometers, bright-field microscopy does not have the resolution to view such small structures. Electron microscopy is required for this purpose.

Summary: Viruses are usually nanometers in size, and their structure is best viewed through the use of electron microscopy.