A is a more complex configuration. It contains steel reinforcement in both the tension zone and the compression zone.
The choice between a singly reinforced and a doubly reinforced beam is —it is a technical decision based on: A is a more complex configuration
Because concrete cannot reliably resist this tension, is placed in the tension zone. This is the genesis of "Reinforced Concrete." A is a more complex configuration
Here is a deep dive into the technical and practical differences between the two. What is a Singly Reinforced Beam? A is a more complex configuration