In 'B' model of DNA, the diameter is 20Å. It is ______in Z DNA.

Options

A. 23Å
B. 18Å
C. 21Å
D. 26Å

Answer

To solve the question regarding the diameter of Z DNA in comparison to B DNA, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Question**: The question asks for the diameter of Z DNA when the diameter of B DNA is given as 20 Å (angstroms).

2. **Know the Forms of DNA**: There are different forms of DNA, primarily A, B, and Z. The question specifically mentions B and Z forms.

3. **Identify the Diameter of B DNA**: From the information provided, we know that the diameter of B DNA is 20 Å.

4. **Research the Diameter of Z DNA**: Now, we need to find out the diameter of Z DNA. According to the comparison of the two forms of DNA:
- The diameter of Z DNA is approximately 18 Å.

5. **Fill in the Blank**: Since the question asks for the diameter of Z DNA, we can fill in the blank with the value we found. Therefore, it is 18 Å in Z DNA.

6. **Select the Correct Option**: The options given were 23 Å, 18 Å, 21 Å, and 46 Å. Since we determined that the diameter of Z DNA is 18 Å, the correct answer is 18 Å.

### Final Answer:
The diameter of Z DNA is **18 Å**.

---