Seven children are to be seated in seven chairs arranged in a row that runs from west to east. All seven children will face north. Four of the children are boys: Frank, Harry, Ivan, and Joel. Three are girls: Ruby, Sylvia, and Thelma. The children are assigned to chairs according to the following conditions:
Exactly one child sits in each chair.
No boy sits next to another boy.
Ivan sits next to and east of the fourth child in the row.
Sylvia sits east of Ivan.
Frank sits next to Ruby.
If Frank sits east of Ruby, which one of the following pairs of children CANNOT sit next to each other?