On cross-motions for judgment on the administrative record, the CFC found the CO reasonably exercised his discretion in finding the coal unacceptable. Whether the coal was an acceptable asset was an issue of responsibility to be determined by the CO in accordance with the principles of FAR Subpart 9.1 and FAR 28.203, which requires a CO to reject an offeror as nonresponsible when the offeror proposes to use a surety whose bid guarantee is unacceptable, and gives the CO discretion in determining whether pledged assets are acceptable. Although the phrase "may be provided" in FAR 28.203-1(b) creates "some degree of ambiguity," when FAR 28.203-1(b) is read in conjunction with FAR Subpart 28.2 as a whole, the provision requires pledged assets other than real property to be deposited into an escrow account in order to be acceptable. The requirement for an escrow account is illustrated by the language of Standard Form 28 and the contract clause at FAR 52.228-11, both of which are required to be executed by individual sureties under FAR Subpart 28.2. To the extent the escrow requirement could be interpreted differently, any ambiguities were patent and thus should have been raised by the contractor prior to submitting its bid. Finally, FAR 28.203-4 permits, but does not require, the CO to allow substitution of assets. Here, the CO reasonably denied substitution based on the fact the protester's surety never submitted a written request for substitution of assets as required by FAR 28.203-4. (Tip Top Construction, Inc. v. U.S., FedCl, 52 CCF ¶78,977)