If Z is open, which of the following must be true ? 

The mixing vat in a factory receives liquid ingredients through six separate valves -- labeled R, S, T,

U, Y and Z - each of which has exactly two settings: Open and closed. The mixing vat operator must

 ensure that each valve is set open or closed according to the following conditions: 

1. If T is open, both S and Z must be closed;                    2. R and Z cannot both be closed at the same time; 

3. If Y is closed, Z must also be closed;                            4. S and U cannot both be pen at the same time.