Problem 4.1 A. Given the X' schema, an adjunct to the X category cannot immediately precede a complement in a phrase with the order X > Complement. Illustrating the point with VP, this order would cause the adjunct to fall between V and NounPhr. Since these nodes are sisters, this could only happen if the adjunct adjoined to V, but we only allow adjunction to V'. In an OV language, on the other hand, adjunction to V' will produce the order Adjunct > Object > V, with the adjunct immediately preceding the object NounPhr.
B. Two adjuncts cannot be sisters under the phrase structure system assumed in the text. Since adjuncts are introduced by tree fragments of the form given below, every adjunct will be a sister of X' and will have no other sisters.