119. Read the following passage, then answer the question below.

The discovery that light can generate electricity dates to 1839, when the French physicist Edmond Becquerel noticed that certain materials produced a small current when exposed to sunlight. For more than a century, however, the effect remained a laboratory curiosity. The turning point came in 1954, when researchers at Bell Laboratories built the first practical silicon solar cell, which converted about six percent of the sunlight striking it into electricity. At that efficiency and cost, the cell could not compete with coal or oil on Earth, but it found a perfect niche in space. In 1958, Vanguard 1, a small American satellite, became the first to be powered by solar cells, and it continued to transmit long after its chemical batteries had died.

On Earth, the economics changed slowly and then suddenly. Mass production in factories, particularly in China, and steady improvements in design reduced the price of solar panels by roughly ninety percent between 2010 and 2020. In many sunny regions, electricity from new solar farms is now cheaper than electricity from new fossil-fuel plants. Commercial panels convert around twenty percent of sunlight to electricity, and laboratory cells have gone much further.

Yet cheap panels do not by themselves solve the problem. The Sun does not shine at night, and in many places solar output peaks at midday, when demand is not highest. Grid operators describe the resulting shape of daily demand, once the solar contribution is subtracted, as a 'duck curve,' with a deep belly at noon and a steep neck in the evening as the panels fade and people return home. Meeting that evening surge requires batteries, flexible power plants, or lines that carry electricity from places where the Sun is still shining. For this reason, many analysts conclude that the future of solar power depends less on the panels than on the storage and the grid that surround them.

What is the 'duck curve' used to describe?