Adenine and thymine are held together by............... hydrogen bonds, while guanine and cytosine are linked by ................. hydrogen bonds.

Explanation

To answer the question regarding the number of hydrogen bonds between adenine and thymine, and guanine and cytosine, we can follow these steps:

### Step-by-Step Solution:

1. **Identify the Base Pairing in DNA**:
- In DNA, there are specific base pairs: adenine (A) pairs with thymine (T), and guanine (G) pairs with cytosine (C).

2. **Determine the Number of Hydrogen Bonds**:
...