...NEW YORK (Standard & Poor's) Sept. 3, 2015--Standard & Poor's Ratings Services today said it assigned its 'A+f' fund credit quality and '##' volatility ratings to the iShares iBonds Dec 2021 AMT-Free Muni Bond ETF and iShares iBonds Dec 2022 AMT-Free Muni Bond ETF. The funds are among more than 220 investment portfolios of the iShares Trust. The trust was organized as a Delaware statutory trust on Dec. 16, 1999, and is authorized to have multiple series or portfolios. The trust is an open-end management investment company registered under the Investment Company Act of 1940 as amended. The offering of the trust's shares is registered under the Securities Act of 1933 as amended. The shares of the trust are listed and traded at market prices on national securities exchanges. BlackRock Fund Advisors (BFA), the fund's investment adviser, is a subsidiary of BlackRock Inc. As of June 30, 2015, BlackRock's assets under management totaled $4.72 trillion across equity, fixed income, cash management,...