The price of the C-fund is at the mercy of just five big tech companies that carry a significant amount of the S&P 500's value. Microsoft, Nvidia, Apple, Alphabet, and Tesla are responsible for more than half the gains in the S&P 500 since April.

"If those companies, for whatever reason, stop performing, there’s nothing to support the market,” said Peter Cecchini, director of research at hedge fund Axonic Capital.
The rest of the market has struggled in the last quarter as we've seen by the S-fund performance.

the share of New York Stock Exchange stocks closing above their 200-day moving averages, dropped as low as 41% earlier in December, a 17-month low.
However, the C-fund has reached new all-time highs in recent week and hasn't strayed too far from those highs. Don't forget the power these big tech companies have over the S&P 500 and its success is dependent on their performance. Be aware that these companies are essentially what you are buying with the C-fund.

Five Big Tech Stocks Are Driving Markets. That Worries Some Investors.