og 256 available reading 是什么意思？

256. The nephew of Pliny the Elder wrote the only eyewitness account of the great eruption of Vesuvius in two letters to the historian Tacitus.

(A) The nephew of Pliny the Elder wrote the only eyewitness account of the great eruption of Vesuvius in two letters to the historian Tacitus.

(B) To the historian Tacitus, the nephew of Pliny the Elder wrote two letters, being the only eyewitness accounts of the great eruption of Vesuvius.

(C) The only eyewitness account is in two letters by the nephew of Pliny the Elder writing to the historian Tacitus an account of the great eruption of Vesuvius.

(D) Writing the only eyewitness account, Pliny the Elder’s nephew accounted for the great eruption of Vesuvius in two letters to the historian Tacitus.（E）

(E) In two letters to the historian Tacitus, the nephew of Pliny the Elder wrote the only eyewitness account of the great eruption of Vesuvius.

E, the best choice.In D, Writing the only eyewitness account, Pliny the Elder’s nephew accounted is redundant, and the placement of in two letters to the historian Tacitus generates ambiguity, suggesting under one available reading that the eruption took place in the letters.

under one available reading 是什么意思呢，怎么也想不出来。

谢谢！

D makes it seems like besides Pliny the Elder’s nephew, there is no other available reading.

His letters are the ONLY evidence

但這是違反暸本來的意思