**Core Concept**
The binding of gp120, a glycoprotein on the surface of the HIV virus, to the CD4 receptor on the surface of T cells plays a crucial role in the initial stages of HIV infection. This binding event triggers a series of subsequent interactions that facilitate the entry of the virus into the host cell.

**Why the Correct Answer is Right**
The binding of gp120 to CD4 triggers a conformational change in gp120, exposing a binding site for a co-receptor, typically CCR5 or CXCR4. This co-receptor is essential for the subsequent steps of viral entry, as it allows the virus to fuse with the host cell membrane. The interaction between gp120 and the co-receptor is a critical step in the HIV life cycle, as it enables the virus to infect the host cell.

**Why Each Wrong Option is Incorrect**
**Option A:** Infection of target cell - While the binding of gp120 is a step towards infection, it is not the direct cause of infection. Infection occurs after the virus has fused with the host cell membrane.

**Option C:** Fusing of virus and target cell - The fusion of the virus with the host cell membrane occurs after the binding of gp120 to the co-receptor, not as a direct result of gp120 binding to CD4.

**Option D:** None - This option is incorrect because gp120 binding does have a significant effect on the subsequent steps of viral entry.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the binding of gp120 to CD4 triggers a cascade of events that ultimately lead to viral entry. Understanding this process is crucial for the development of effective HIV therapies that target these early stages of infection.

**✓ Correct Answer: B. Facilitation of co-receptor**