4. Girls have an xx chromosome. What do boys have? a BX (B) XY xx

## Step 1<br />The problem is about human genetics, specifically the sex chromosomes. In humans, sex is determined by the combination of sex chromosomes that an individual inherits from their parents. <br /><br />## Step 2<br />Females typically have two X chromosomes, represented as XX. This is the standard genetic makeup for females.<br /><br />## Step 3<br />Males, on the other hand, have one X chromosome and one Y chromosome, represented as XY. This is the standard genetic makeup for males.<br /><br />## Step 4<br />The presence of the Y chromosome determines male sex characteristics. Therefore, the correct answer to the question "What do boys have?" is (B) XY.