Streamlit’s `st.selectbox` displays a labeled list of options and returns the selected value. By default, the first option is selected; use `index` to choose a different initial option. section 3.6.1
In a unit converter, dropdowns can:
- Let users choose the units to convert from and to, with options updated for the selected quantity. section 3.5.3
- Show From and To choices side by side; a different initial selection can avoid starting with the same unit in both. section 3.6.1
Figure 3.10 Unit Converter app after adding From and To dropdowns
See the unit converter interface with From and To dropdowns. section 3.6.1
