Select the principal group when the following groups are present in a molecule.
`-SH ,-I ,_CONH_2`

Explanation

To determine the principal functional group among the groups -SH (thioalcohol), -I (iodine), and -CONH2 (amide), we need to follow the priority order of functional groups as per IUPAC nomenclature.

### Step-by-Step Solution:

1. **Identify the Functional Groups**:
- -SH is a thioalcohol (also known as a thiol).
- -I is a halogen (specifically iodine).
- -CONH2 is an amide.
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