Join the following sentences to make one complete sentence without using and, but or so.
Sarah and Tyra are twins. They look exactly alike.
Topic: Synthesis — clause of reason
Answer
Sarah and Tyra look exactly alike, as they are twins. (Also acceptable: Being twins, Sarah and Tyra look exactly alike.)
The relationship between the two sentences is CAUSE AND EFFECT, and the direction matters: being twins is the REASON they look alike, not the other way round. So AS or BECAUSE introduces the twin clause.
The participial version — "Being twins, they look exactly alike" — is shorter and equally correct, and is worth knowing because it avoids a conjunction altogether, which is exactly what the instruction asks for.
METHOD for every synthesis question: identify the relationship first (reason, contrast, time, condition), then check its DIRECTION before choosing the conjunction.