I have come across a question in a textbook which asks to identify how many stereo isomers are possible for a displayed organic compound (attached below).
The answer states that there would be 4 stereoisomers and gives the following explanation: "The stereoisomers with the skeletal formula shown will all be Z isomers. There will be (+) and (−) isomers at both the chiral centres ; therefore four stereoisomers; with the skeletal formula shown."
I do not understand why the skeletal formula shows will only be z isomers? Does anyone understand this?