Give the decreasing order or reactivity for the elimination with strong bases of `RX(X=F,CI,Br, `and `I). `
`b.` How is this information important to decide between `E2` and `E1cB`?

Explanation

To determine the decreasing order of reactivity for the elimination reactions of RX (where X = F, Cl, Br, I) with strong bases, we need to consider the bond strengths of the C-X bonds. The reactivity in elimination reactions, particularly E2 reactions, is influenced by the strength of the bond between carbon and the leaving group (X).

### Step-by-Step Solution:

1. **Identify the Leaving Groups**: The leaving groups in this case are F (Fluorine), Cl (Chlorine), Br (Bromine), and I (Iodine).

2. **Analyze Bond Strengths**: The reactivity of the leaving groups in elimination reactions is inversely related to the bond strength of the C-X bond. The weaker the bond, the better the leaving group, and thus the more reactive it is in elimination reactions. The bond strengths generally decrease in the order:
- C-F > C-Cl > C-Br > C-I
...