The solution that is optically inactive but chiral with 4 substituents is a meso compound. A meso compound is a molecule with chiral centers but also has an internal plane of symmetry, which results in the overall molecule being optically inactive. This occurs when the substituents on the chiral centers cancel each other out in terms of their effect on the overall molecule's chirality. Therefore, even though the molecule is chiral, it does not rotate plane-polarized light and is optically inactive. **

In a Fischer projection, the substituents to the left represent the groups that are pointing towards the viewer, while the substituents to the right represent the groups that are pointing away from the viewer. This convention helps to visualize the three-dimensional structure of a molecule in a two-dimensional representation. **

In a Fischer projection, the substituents on the left side represent the vertical bonds, while the substituents on the right side represent the horizontal bonds. This convention helps to easily identify the stereochemistry of the molecule and determine the relative positions of the substituents. The horizontal bonds on the right side are typically pointing towards the viewer, while the vertical bonds on the left side are pointing away from the viewer. **

In a Fischer projection, the substituents that are located to the left are typically the ones with higher priority according to the Cahn-Ingold-Prelog rules, such as groups with higher atomic numbers or multiple bonds. The substituents on the right are usually those with lower priority, like hydrogen atoms or groups with lower atomic numbers. The arrangement of substituents in a Fischer projection helps to determine the stereochemistry of a molecule. **

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Trans-2-pentene can have both CH3 substituents below on the same side because the trans configuration refers to the orientation of the two highest priority groups on the double bond, which in this case are the two CH3 groups. In trans-2-pentene, the two CH3 groups are on opposite sides of the double bond, with the other substituents (H atoms) also on opposite sides. This results in a trans configuration, even though the CH3 groups are both below on the same side. **

Even though the CH3 substituents are both pointing downwards on the same side, the molecule can still be trans-2-pentene. In trans-2-pentene, the double bond is between the second and third carbon atoms, and the CH3 groups are on the same side of the double bond. This arrangement results in a trans configuration, where the two CH3 groups are on opposite sides of the double bond, even though they are both pointing downwards. **