Choose A. Second-degree atrioventricular block is divided into two types: Type I, also known as Venn phenomenon, is the most common type of second-degree atrioventricular block. Expressed as: 1 interval. PR gradually prolonged until 1 P wave was blocked and could not be transmitted to the ventricle; 2. Adjacent RR intervals gradually shorten until the 1 P wave can't pass down to the ventricle; 3. RR interval including blocking P wave is less than twice that of normal sinus PP interval. In addition: 1. The most common conduction ratio is 3: 2 or 5: 4, so it is a B fault. 2.2 There are many reasons. QRS complex falls off, which is not its characteristic, so C is not selected. 3. Option D is 3 degree atrioventricular block. 4. Slow conduction in any part of atrioventricular conduction bundle can prolong PR interval, so E is not selected.