原文：
credit bullets in conjunction with long-end treasury structures are used in a barbell strategy,Callable bonds provide a spread premium that can be valuable to an investor during periods of high interest rate volatility,put strutures will provide investors with some protection in the event that interest rates rise sharply but not if the issuer has an unexpected credit event.
问最正确的是哪个，答案是bullet
但是bullet的这个描述我也没看太懂。。。
然后call的我猜是option premium 错在这里
put的感觉没错
求解答