What is true about guards on interaction operands?

What is true about guards on interaction operands?
A . should not refer to any dynamic variables, i.e., to attributes of the lifelines
B . must only cover one lifeline, namely the one lifeline having the first event occurrence of the interaction operand
C . must always cover all lifelines covered by the operand
D . must cover the same lifelines on all interaction operands within one combined fragment

View Answer

Answer: B